Notify 473: add code coverage for jest (#488)

tests/javascripts/jest.config.js

@@ -1,7 +1,17 @@
 module.exports = {
+  collectCoverage: true,
+  coverageDirectory: './coverage',
+  coverageThreshold: {
+    global: {
+      branches: 75,
+      functions: 90,
+      lines: 90,
+      statements: 90,
+    }
+  },
   setupFiles: ['./support/setup.js'],
   testEnvironment: 'jsdom',
   testEnvironmentOptions: {
     url: 'https://www.notifications.service.gov.uk',
   },
-}
+};