Purify the logic of test mapping.

Interestingly, the comment for `atest_utils.is_test_mapping` mentioned
the flag `--smart-testing-local`, but has not filled in the logic.

Bug: 406320968
Test: atest-dev atest_unittests
Change-Id: Ifbc1633b6b6b223feb72196f30f619a681a00f8a
2 files changed