Implement `TEST_PREMATURE_EXIT_FILE`

[fmeum]

The Bazel test docs claim that a test will fail if a file exists at the path indicated by TEST_PREMATURE_EXIT_FILE, but this has not yet been the case in Bazel.

This feature is used by googletest (https://google.github.io/googletest/advanced.html#detecting-test-premature-exit).

[fmeum]

@lberki Is this feature supported by the internal test strategy implementation? Could you also let me know what test.warnings is about?

[fmeum]

@lberki Friendly ping