Consolidate integration test introduction pages

[parlough]

We have two integration test pages that have some overlap: Integration testing and An introduction to integration testing.

I think we should:

1. Consolidate the good intro and task-based content into one document. Call that "Introduction" under "Integration testing" in the sidenav. Don't include the Firebase Test lab content.
2. Make a redirect from the other page to that one.
3. For the Firebase test lab content, either:
   1. Create a new page for it, link to it from the sidenav
   2. Determine if Firebase has sufficient replacement documentation and link to that from the introduction page instead of having our own.