test: Kill weston whenever shell exits

This script runs with -e, so if the timeout command returns a non-0 exit status (meaning the while loop timed out), the script exits immediately as well.

This would leave weston running in the background, which resulted in meson waiting for weston to terminate until hitting meson's own timeout.

Instead, explicitly kill weston whenever the shell exits. This results in meson recording the test as failed immediately.

As a bonus, we can drop the special handling around run-piglit.sh.

Plus 3 related minor improvements.