gtester: fix -Wduplicated-branches
gtester: fix -Wduplicated-branches

One of the “quick” or “slow” test run modes is always added to the
argument list, making the branching pointless, which, coincidentally,
now causes a warning.

https://bugzilla.gnome.org/show_bug.cgi?id=793399

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib