Restore all mocks after each test

This is easier than re-assigning the mock functions manually, as
we're reusing Jest's in-built behaviour. Because all the mocks
are restored, we need to move the ones we had in the beforeAll
block into the beforeEach block.

Note: "require('./support/teardown.js')" also resets all Jest
mocks, but "require" only runs once, so we can't use it in a
beforeEach block [1]. We could do a "jest.resetModules()" to fix
that, which seems worse on the whole. I think there's a broader
discussion here about whether we could / should have a global
reset of Jest mocks after each test - I quickly tried this and
it causes some existing tests to fail :-|.

[1]: https://stackoverflow.com/questions/48989643/how-to-reset-module-imported-between-tests

tests/javascripts/registerSecurityKey.test.js

@@ -2,13 +2,6 @@ beforeAll(() => {
   window.CBOR = require('../../node_modules/cbor-js/cbor.js')
   require('../../app/assets/javascripts/registerSecurityKey.js')
 
-  // disable console.error() so we don't see it in test output
-  // you might need to comment this out to debug some failures
-  jest.spyOn(console, 'error').mockImplementation(() => {})
-
-  // ensure window.alert() is implemented to simplify errors
-  jest.spyOn(window, 'alert').mockImplementation(() => {})
-
   // populate missing values to allow consistent jest.spyOn()
   window.fetch = () => {}
   window.navigator.credentials = { create: () => {} }
@@ -26,6 +19,13 @@ describe('Register security key', () => {
   let button
 
   beforeEach(() => {
+    // disable console.error() so we don't see it in test output
+    // you might need to comment this out to debug some failures
+    jest.spyOn(console, 'error').mockImplementation(() => {})
+
+    // ensure window.alert() is implemented to simplify errors
+    jest.spyOn(window, 'alert').mockImplementation(() => {})
+
     document.body.innerHTML = `
       <a href="#" role="button" draggable="false" class="govuk-button govuk-button--secondary" data-module="register-security-key">
         Register a key
@@ -35,6 +35,10 @@ describe('Register security key', () => {
     window.GOVUK.modules.start()
   })
 
+  afterEach(() => {
+    jest.restoreAllMocks()
+  })
+
   test('creates a new credential and reloads', (done) => {
 
     jest.spyOn(window, 'fetch').mockImplementationOnce((_url) => {