libnm-glib: make test-networkmanager-service.py automatically exit with its parent

libnm-glib: make test-networkmanager-service.py automatically exit with its parent

test-nm-client.c and test-remote-settings-client.c were using their
own assertion macros so they could kill the test service on assertion
failure. Except that some new code didn't get the memo and used the
g_assert* macros. Not to mention that sometimes the tests would crash
outside of an assertion macro.

We can make test-networkmanager-service.py notice that its parent has
crashed by opening a pipe between them and taking advantage of the
fact that the pipe will be automatically closed if the parent crashes.
So then test-networkmanager-service.py just has to watch for that, and
exit if the pipe closes.

Then that lets us drop the test_assert* macros and just use g_assert*
instead.