test: Unset JOURNAL_STREAM for integration test units

We use StandardOutput=journal+console for integration test units
to make sure the output from the test ends up on both the serial
console and in the journal. Unfortunately this only works for
messages written to stdout/stderr of the test service. All journal
messages logged by systemd tools will only end up in the journal and
not on the serial console. This makes test failures harder to debug
than needed because developers have to fetch the journal to figure
out the actual error that was logged by any systemd tool that failed
when invoked from the integration test script.

To fix this, let's unset $JOURNAL_STREAM for the integration test
units. This forces all output to go through stdout, which will make
journald forward it to the console and save it in the journal.

test/test.service.in

@@ -10,3 +10,4 @@ ExecStart=@command@
 Type=oneshot
 MemoryAccounting=@memory-accounting@
 Environment=SYSTEMD_PAGER= @env@
+UnsetEnvironment=JOURNAL_STREAM