test: kill all processes launched by test-execute before exiting

test: kill all processes launched by test-execute before exiting

As was shown in https://github.com/systemd/systemd/issues/10696#issuecomment-439613204,
currently `meson` waits for 1080 seconds (which is three times the global timeout) for the
test to fail completely even though it takes just two minutes for it to really fail. This
happens because the test itself leaves the services it has launched behind, which, in turn, makes
meson think that the test is still in progress. KILL_ALL with SIGKILL should make the issue
go away.