TEST-#7441: Correctly skip sanity tests if we don't need them.

[sfc-gh-mvashishtha]

The "sanity" tests for ray, dask, and unidist are supposed to run a subset of the "test-all" suite. If we detect a change to a particular engine, we're supposed to run the test-all job using that engine. However, if we don't detect a change, we're supposed to run the "test-sanity" job using that engine.

Prior to this commit, we always run the sanity jobs because the if key-value pairs here don't do anything. In this commit, use test matrix exclusions to skip executions that we don't need to run.

Testing

I have validated that we only run the expected sanity tests in the following scenarios, which are older commits on the branch I am trying to merge here:

• change ray, dask, and unidist => skip all sanity tests

• change ray and unidist only => run dask sanity test only

• change dask and ray only => run unidist sanity tests only

• change dask and unidist only => run ray sanity tests only

• change unidist only => run ray and dask sanity tests only

• change ray only => run unidist and dask sanity tests only

• change dask only => run ray and unidist sanity tests only

• first commit message and PR title follow format outlined here

  NOTE: If you edit the PR title to match this format, you need to add another commit (even if it's empty) or amend your last commit for the CI job that checks the PR title to pick up the new PR title.

• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py

• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py

• signed commit with git commit -s

• Resolves TEST: CI never skips sanity tests #7441

• tests added and passing

• module layout described at docs/development/architecture.rst is up-to-date