tests: add racing tests

[cyphar]

We need to add some tests (like those in libpathrs) to test that our implementations are safe against racing attackers (this should already be the case by design, but we really should verify it in our test suite).

This is separate to #6 because it doesn't really make much sense to do the race tests in the binding-agnostic tests (if the core of libpathrs is race-safe then the simple binding wrappers will also be race-safe, and it will be easier to do the tests in libpathrs itself).