Tests for #9284, FUSE client capability handling

I'm still not getting how these filenames work.
We've got abs_path, which is going to be constructed from mountpoint (eg "/tmp/cephtest/mnt.1") and the fs_path (eg "open_files") for a path on the local machine (eg "/tmp/cephtest/mnt.1/open_files).
Then we create all the directories named in that path. This makes sense.

But then each file is of the form {abs_path}_{N}, where N is a number. Something like /tmp/cephtest/mnt.1/open_files_1.

So we're not actually using the /tmp/cephtest/mnt.1/open_files directory at all — though if our full path was /tmp/cephtest/mnt.1/caps_test/files we would be creating files like /tmp/cephtest/mnt.1/caps_test/files_1, which might be fine. But we're using the final part of the fs_path name not as a directory but as a prefix.

Am I missing something about this?