Enable test fixtures and migrate datastore test setup

[emartynov]

What I have done

I've enabled Kotlin test fixtures and converted test-related modules to test fixtures:

1. Added flag in gradle.properties
2. Configured test fixtures in feature module
3. Moved code from the test module to the test fixtures folder in the feature module
4. Replaced all references from test module with test fixtures from feature module
5. Deleted the test module

Why

I believe test fixtures have two advantages from the test module:

1. It doesn't need to be a separate module, and the test fixture code lies close to the definition of dependency that needs to be set in tests.
2. Not for this change, but test fixtures have access to internal code in the module, making the public module API cleaner and smaller.

Further changes

• Put official documentation about the feature.
• Migrate a couple of other test-related modules to test fixtures.

[SimonMarquis]

FYI some work was already been done in:

• [FR]: Migrate project to testFixtures #451
• TestFixtures refactoring #1515

But we are waiting on the maintainers feedback here and there

[emartynov]

Ah, I didn't notice it, sorry! Let me see PRs.

And I think I will close it.

[emartynov]

Closing since there is full set in #1515