relesetools: Support running all the unittests with atest.

All the unittests will be built into releasetools_test. One can run the
tests with `atest releasetools_test` or the traditional way
`test_utils.py`. The atest way is recommended, which additionally builds
the required tools.

With the current support in Soong, we can't pack the built tools into
releasetools_test yet. So running `releasetools_test` alone in clound
would fail. Follow-up CLs will address the issue in order to deploy the
tests with TEST_MAPPING.

Bug: 112080715
Test: `atest releasetools_test`
Change-Id: Ica95517a5ab326f4e58fc57c6c2c276cfe882f3c