fix(launcher): cancel kill timeout when process exits cleanly

By default browsers are killed with `process.kill()` but it might
happen that a process doesn't play nicelly and doesn't exit so
there is a timeout after which a process is SIGKILL-ed. Before
this commit the mentioned timeout wasn't cleared on clean process
exit, preventing Karma from exiting just after all tests were run.

Fixes #946
Merge branch 'pr-1054' into presubmit-stable-pr-1054
chore: release v0.12.16