Establish new testing workflow

[dchigarev]

Since Modin will have many different backends, we should establish a testing process for all of them.

After static API for query compilers will be established we should try to implement this concept:

1. Run current API tests (tests from modin/pandas/test) only with BaseQueryCompiler, because all of its methods do fallback to pandas and pretends to work correctly, so if some tests is failed, we can be sure that something is wrong at the API layer part.
2. Write a common test base that will test every query compiler with common tests, to check that every query compiler satisfies our established query compilers API and works correctly
3. Provide the ability to backend maintainers to easily adding backend-specific tests.

This testing workflow allows us to test everything without unnecessary retesting