Require that all a classes that should have tests, have a corresponding test file (& in the right location) [Blocked: #59, #1724]

[BenHenning]

Is your feature request related to a problem? Please describe.
Code can be introduced today without tests.

Describe the solution you'd like
In order to ensure the app has good testing coverage, we should require all new code have test files present. Later work can ensure that code coverage itself is properly met.

In particular, I think we should ensure there's an exact 'Test' file corresponding to each new file, but in the test directory. Unfortunately, we cannot do this until #59 (Bazel migration stage 2) is completed since that will reorganize the project into its final directory structure. There are also some files which probably do not require dedicated tests since they would be trivial/change detector tests that are covered by other integration tests:

• Components (which will eventually be automatically generated once Add support for Dagger Hilt [Blocked: #59] #1720 is completed)
• Modules
• Data structure only classes (if these even exist--most pure data structures are represented via protos)
• Presenters (they should share a test with their fragment/activity/view counterpart)

[BenHenning]

This was fixed in #3292.