test: make transient hostname tests fail verbosely (#4754)

This test fails sometimes but it is hard to reproduce, so we need more
information what happens. Set journal log level to "debug" for the entirety of
networkd-test.py, and show networkd's and hostnamed's journals and the DHCP
server log on failure of the two test_transient_hostname* tests. Also sync the
journal before querying it to get more precise output.

This should help with tracking down issue #4753.