Store built librt under build/librt-cache, so that it can be reused to speed up mypyc tests when there are no changes under mypyc/lib-rt. Also invalidate the cache if the build environment changes.

This also ensures that each test run only compiles librt at most twice (with and without experimental features), instead of once per test case that requires librt.

Use filelock to support parallel tests.

Building librt can take 6+ seconds, so the savings can be quite significant. We are also planning to add several additional librt submodules, which will further slow down the build, and this will result in a larger number of tests that require librt. The main benefit is that adding more tests or submodules doesn't significantly slow down tests.