commandtest: Use virTestCompareToFile() in checkoutput()

commandtest: Use virTestCompareToFile() in checkoutput()

In the commandtest there is checkoutput() function which checks
the latest log of commandhelper (containing things like cmd line
arguments, env vars, FDs, CWD, etc.) and compares that against
expected output. Well, the way this function implements that is
effectively by open coding virTestCompareToFile() except for the
nice feature that the virTestCompareToFile() has:
VIR_TEST_OUTPUT_REGENERATE.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>