In the core BC policy (https://www.drupal.org/core/d8-bc-policy) it states :
Tests
The contents of automated tests provided by core modules are never considered part of the API. While the testing framework itself may be treated as an API individual test classes are always internal.
I propose that we reword this to exclude the entire testing framework from the semver BC promise but instead promise a best effort not break contributed tests running on Drupal.org.
This has come up in #2469721: Add 'verbose' functionality to BrowserTestBase
Rationale
The Testing framework requires development that won't be the focus during main product development therefore aligning it to the product semver promise effectively hamstrings innovation in this idea and results in major disadvantages. The only advantage of keeping the semver promise is that contrib module's test don't break because of a core testing API change. However I think that we can cover this with a looser promise to make best efforts to not break contributed tests running on Drupal.org.