Also check that FileNotFound is consistently returned when the path is missing.
The new `run_relative` step will test spawning paths like:
child_path: ../84385e7e669db0967d7a42765011dbe0/child
missing_child_path: ../84385e7e669db0967d7a42765011dbe0/child_intentionally_missing
67d5bfef removed std.ChildProcess tests, suggesting to make them
standalone instead. This commit does exactly that after the
bug creating SIGPIPE in ReleaseFast is no more with LLVM 15.0.5.
Thanks to @x1ddos for the idea with the compile artifacts and PR
improvements.