Parallelize running integration tests with rayon

[fschutt]

Right now in #3353 we are running integration tests with a single thread, which is especially slow when running the create-exe tests because of compilation. We should run these tests in parallel, however the last time I tried doing that I ran into problems with static_defs.h being written to from multiple threads (race condition on disk). We should switch to temp directories when refactoring the create-exe code, then re-enable rayon.

[Michael-F-Bryan]

It'll be easier to work on this after #3430 is merged.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Feel free to reopen the issue if it has been closed by mistake.