Tests for #9284, FUSE client capability handling

Nope, this is creating subdir/mount_a_1, subdir/mount_a_2 etc.  The optional subdir is just to cover the different behaviour in Client for recalls in root vs. elsewhere in FS -- it's not meant to be labelling the subdirs by mount.  "mount_a" here could be "foo", it's just "mount_a" because I was reminding myself while writing this that these files were created by mount_a.