tests: Wait for gdbus-testserver to die when killing it

This was previously done (by commit 63038d1e) in one of the cases where kill_test_service() was called — but not the other.

This meant that one instance of gdbus-testserver could still be around when (as it happens, due to the order of the tests) the /gdbus/proxy/no-match-rule test was run. It would start a second instance of gdbus-testserver, which would exit early due to the test name still being owned on the bus. The first (killed) instance of gdbus-testserver would then exit, leaving no test servers running, and hence the new test would fail.

This was being seen as frequent CI failures, particularly on FreeBSD (must have slightly different timing for process signalling and termination from Linux).

Signed-off-by: Philip Withnall pwithnall@endlessos.org