functional: improve TestReconfigureServer test
functional: improve TestReconfigureServer test

Remove the sleep call that allowed us to produce reliable tests, it
turns out it may fail in rare cases. I suspect this due to journald
delays and how things are serialized and started when running the test.

Instead, start a normal unit then unload it to reproduce a real behaviour
then follow up with the SIGHUP on fleetd.

If the "Reloading configuration" message do not show up on the logs, then
we do not fail since the signal was received and processed, instead
continue with the "list-units" check, if it succeed then the test should have
passed at this point. However, we continue and check the logs that we ignored
previously since the test also checks for the
"Failed serving HTTP on listener" which informs us that fleetd failed.
So we check the logs again if we find them we proceed with that message
and check it, otherwise we skip the test at this point. We do not want
developers to have failing functional tests results due to obscure
delays. However the test succeed now and is more reliable. I have been running
it for more than 1 hour now (more than 100 times), it did not fail and it was
never skipped.

jonboulle/fleet,endocode/fleet,giantswarm/fleet,giantswarm/fleet,dongsupark/fleet,jonboulle/fleet,jonboulle/fleet,endocode/fleet,dalbani/fleet,dalbani/fleet,coreos/fleet,endocode/fleet,dongsupark/fleet,dalbani/fleet,dongsupark/fleet,coreos/fleet,giantswarm/fleet,coreos/fleet