Let's keep the debug logs in the journal, while logging only
testsute-*.sh stdout/stderr to the console (ba7abf7). This should make
the test output log a bit more readable and potentially the tests itself
a bit faster by avoiding console oversaturation.
Also, it should significantly reduce the size of artifacts kept by CIs.
This verifies that the argv part of any exec_command parameters that
are sent through dbus is not empty at deserialization time.
There is an additional check in service.c service_verify() that again
checks if all exec_commands are correctly populated, after the service
has been loaded, whether through dbus or otherwise.
Fixes#20933.