tests: kill worker pids as well on timeouts

Workers that don't exit can also trigger a timeout, and we want our
test environment to be super robust - kill them aggressively too.

Also raise the timeout error so that we don't mask this when it
happens.

Change-Id: Iee606720f98e9a5e8a11f162eb885c0a00a2345e
Partial-Bug: #1357578
Merge "tests: kill worker pids as well on timeouts"