[libc] [UnitTest] Add timeout to death tests

Summary:
This patch adds a timeout of 500ms to death tests. As we add multithreaded code and locks, deadlocks become more likely so timeout will be useful.

Additionally:
 - Better error handling in `invokeSubprocess`
 - Makes `ProcessStatus`'s methods const

Reviewers: sivachandra, MaskRay, gchatelet, PaulkaToast

Reviewed By: sivachandra, PaulkaToast

Subscribers: tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D75651