Tests for #9284, FUSE client capability handling

> Am I missing something about this?

Yes :-)  I think you're missing the dirname() call that's in the directory creation: for e.g. "open_files", the abspath is e.g. "/tmp/cephtest/mnt.1/open_files", the dirname of which is "/tmp/cephtest/mnt.1".  So in that case we don't create any directory because it already exists -- the open_files_{n} files are created in the root of the filesystem.

For "subdir/mount_a", abs_path is something like "/home/