The prior method was only checking for a PID's existance. We should
also check for zombie process states. This allows our child process
launcher to correctly determine when a process is done on Linux and
other Posix systems.

Also starts the test timer on Posix LaunchProcess implementations.

Also removes the need to end command line arg lists with nullptr.

Also removes some Fuchsia-specific preprocessor checks.

Also adds a regression test.

Bug: angleproject:3162
Change-Id: I5725a8a0e8c5151f2d3e06df0a36dd64c0e0ee69
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1928873
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>