syslog: rsyslog doesn't create empty log files

Newer versions of rsyslog don't create empty log files during startup -
the files are created upon first logged entry instead.
(since rsyslog git commit de77494415ae8c169949d13ed0df0af3b1949b54)

Don't check if log file gets created after service start,
only make sure it exists after test generates a new log entry.

Signed-off-by: Jiri Jaburek <jjaburek@redhat.com>
Signed-off-by: Jan Stancek <jstancek@redhat.com>