Support Bazel `TEST_PREMATURE_EXIT_FILE`

[mattyclarkson]

Description

When running bazel test it creates the TEST_PREMATURE_EXIT_FILE which contains an absolute path to a file in a writable directory.

If a test binary creates that file, then removes it on a successful exit it allows the test runner to understand when a test binary has been aggressively aborted.

Additional Context

In order to catch early exit, a test may create a file at the path specified by TEST_PREMATURE_EXIT_FILE upon start and remove it upon exit. If Bazel sees the file when the test finishes, it will assume that the test exited prematurely and mark it as having failed.

Bazel test encyclopedia

[horenmar]

The target path will be limited to target an existing directory. Creating and removing an empty file is easy-enough in C++14, but messing around with directories needs C++17's filesystem.