commandtest: Remove commandhelper.log in test27

The recently added test27 spawns commandhelper. This is fine,
except, one of the things that commandhelper does is it records
arguments it was spawn with into commandhelper.log. Other test
cases then use checkoutput() to compare the arguments against the
expected ones and also unlink() the log file. However, test27()
is not doing that and thus it leaves the file behind. This
breaks distcheck.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>