Fix potential hang in unit/logging_timestamp_test

If Log() fails to deliver even one newline, the fgets() later in the
test shall hang indefinitely.  So make sure we send a newline to
stderr after the Log(), to prevent that.  This shouldn't affect the
validity of the timestamp in what's sent before ...

This change is a pre-requisite for Mark's logging changes, but
is in any case desirable to make the test robust, so that it fails
promply rather than hanging a build slave if someone breaks it.