compiletest: add a `needs-subprocess` directive

[jieyouxu]

This is more semantically meaningful than random e.g. ignore-sgx directives, and if and when some targets do gain subprocess support, it's easier to update all the tests that rely on subprocesses in one go and try to see if they fail.

[jieyouxu]

To implement this, we may need another hacky collection of known allowlist/denylist of subprocess support 🤦

[the8472]

You could probe it by attempting to spawn a process