Refactor integration test helpers

This commit performs three operations:

1. Move `integration-tests/helpers.js` to
   `integration-tests/helpers/index.js`.
2. Move `FakeAgent` from inside `integration-tests/helpers/index.js` to
   its own file called `integration-tests/helpers/fake_agent.js`.
3. Move the setting up of the Express app inside the `FakeAgent.start`
   method to a private helper function.

integration-tests/helpers/index.js

@@ -338,11 +338,6 @@ function sandboxCwd () {
   return sandbox.folder
 }
 
-function base64 (strOrObj) {
-  const str = typeof strOrObj === 'string' ? strOrObj : JSON.stringify(strOrObj)
-  return Buffer.from(str).toString('base64')
-}
-
 module.exports = {
   FakeAgent,
   spawnProc,