gctl_test.t: more tweaks to try and update the code and get it functional (again?)

• Make the logfile for $out be built off the basename for $cmd, instead of $cmd. (r317292 broke this assumption).
• Rename $mntpt to $mntpt_prefix for clarity, as this variable is a prefix for mountpoints.
• Reindent the umount directive block while here to match the rest of the code.

MFC after: 5 weeks
Sponsored by: Dell EMC Isilon