use mockImplementationOnce to define separate return vals for fetch

rather than having a gross if/else, we can define separately. This means
we can separate the asserts and test setups for the first fetch (get)
and the second fetch (post), which means we can arrange all the mocks in
the order they're called in the function, significantly enhancing
legibility of the tests

tests/javascripts/registerSecurityKey.test.js

@@ -36,6 +36,17 @@ describe('Register security key', () => {
   })
 
   test('creates a new credential and reloads', (done) => {
+
+    jest.spyOn(window, 'fetch').mockImplementationOnce((_url) => {
+      // initial fetch of options from the server
+      // options from the server are CBOR-encoded
+      const webauthnOptions = window.CBOR.encode('options')
+
+      return Promise.resolve({
+        ok: true, arrayBuffer: () => webauthnOptions
+      })
+    });
+
     jest.spyOn(window.navigator.credentials, 'create').mockImplementation((options) => {
       expect(options).toEqual('options')
 
@@ -49,29 +60,23 @@ describe('Register security key', () => {
       })
     })
 
+    jest.spyOn(window, 'fetch').mockImplementationOnce((_url, options) => {
+      // subsequent POST of credential data to server
+      const decodedData = window.CBOR.decode(options.body)
+      expect(decodedData.clientDataJSON).toEqual(new Uint8Array([4,5,6]))
+      expect(decodedData.attestationObject).toEqual(new Uint8Array([1,2,3]))
+      expect(options.headers['X-CSRFToken']).toBe()
+      return Promise.resolve({ ok: true })
+    });
+
     jest.spyOn(window.location, 'reload').mockImplementation(() => {
       // signal that the async promise chain was called
       done()
     })
 
-    jest.spyOn(window, 'fetch').mockImplementation((_url, options = {}) => {
-      // initial fetch of options from the server
-      if (!options.method) {
-        // options from the server are CBOR-encoded
-        webauthnOptions = window.CBOR.encode('options')
-
-        return Promise.resolve({
-          ok: true, arrayBuffer: () => webauthnOptions
-        })
-
-      // subsequent POST of credential data to server
-      } else {
-        decodedData = window.CBOR.decode(options.body)
-        expect(decodedData.clientDataJSON).toEqual(new Uint8Array([4,5,6]))
-        expect(decodedData.attestationObject).toEqual(new Uint8Array([1,2,3]))
-        expect(options.headers['X-CSRFToken']).toBe()
-        return Promise.resolve({ ok: true })
-      }
+    // this will make the test fail if the alert is called
+    jest.spyOn(window, 'alert').mockImplementation((msg) => {
+      done(msg)
     })
 
     button.click()
@@ -81,7 +86,7 @@ describe('Register security key', () => {
     ['network'],
     ['server'],
   ])('alerts if fetching WebAuthn options fails (%s error)', (errorType, done) => {
-    jest.spyOn(window, 'fetch').mockImplementation((_url, options = {}) => {
+    jest.spyOn(window, 'fetch').mockImplementation((_url) => {
       if (errorType == 'network') {
         return Promise.reject('error')
       } else {
@@ -102,32 +107,32 @@ describe('Register security key', () => {
     ['internal server error'],
     ['bad request'],
   ])('alerts if sending WebAuthn credentials fails (%s)', (errorType, done) => {
+
+    jest.spyOn(window, 'fetch').mockImplementationOnce((_url) => {
+      // initial fetch of options from the server
+      const webauthnOptions = window.CBOR.encode('options')
+
+      return Promise.resolve({
+        ok: true, arrayBuffer: () => webauthnOptions
+      })
+    })
+
     jest.spyOn(window.navigator.credentials, 'create').mockImplementation(() => {
       // fake PublicKeyCredential response from WebAuthn API
       return Promise.resolve({ response: {} })
     })
 
-    jest.spyOn(window, 'fetch').mockImplementation((_url, options = {}) => {
-      // initial fetch of options from the server
-      if (!options.method) {
-        webauthnOptions = window.CBOR.encode('options')
-
-        return Promise.resolve({
-          ok: true, arrayBuffer: () => webauthnOptions
-        })
-
+    jest.spyOn(window, 'fetch').mockImplementationOnce((_url) => {
       // subsequent POST of credential data to server
-      } else {
-        switch (errorType) {
-          case 'network error':
-            return Promise.reject('error')
-          case 'bad request':
-            message = Promise.resolve(window.CBOR.encode('error'))
-            return Promise.resolve({ ok: false, arrayBuffer: () => message })
-          case 'internal server error':
-            message = Promise.reject('encoding error')
-            return Promise.resolve({ ok: false, arrayBuffer: () => message, statusText: 'error' })
-        }
+      switch (errorType) {
+        case 'network error':
+          return Promise.reject('error')
+        case 'bad request':
+          message = Promise.resolve(window.CBOR.encode('error'))
+          return Promise.resolve({ ok: false, arrayBuffer: () => message })
+        case 'internal server error':
+          message = Promise.reject('encoding error')
+          return Promise.resolve({ ok: false, arrayBuffer: () => message, statusText: 'error' })
       }
     })
 
@@ -144,9 +149,9 @@ describe('Register security key', () => {
       return Promise.reject(new DOMException('error'))
     })
 
-    jest.spyOn(window, 'fetch').mockImplementation((_url, options) => {
+    jest.spyOn(window, 'fetch').mockImplementation((_url) => {
       // initial fetch of options from the server
-      webauthnOptions = window.CBOR.encode('options')
+      const webauthnOptions = window.CBOR.encode('options')
 
       return Promise.resolve({
         ok: true, arrayBuffer: () => webauthnOptions