X wcfs: tests: Run `fusermount -u` the second time if we had to kill wcfs

This makes sure to cleanup /proc/mounts from stale / broken FUSE
connection, and removes uninformational `assert not is_mountpoint` from
raising, thus, adding more noise in already very verbose wcfs-kill-dump.

wcfs/wcfs_test.py

@@ -247,10 +247,19 @@ class tWCFS(_tWCFS):
             assert not is_mountpoint(t.wc.mountpoint)
             os.rmdir(t.wc.mountpoint)
         defer(_)
+        @func
         def _():
             # kill wcfs.go in case it is deadlocked and does not exit by itself
             if procwait_(timeout(), t.wc._proc):
                 return
+
+            # run `fusermount -u` the second time after we kill wcfs to cleanup
+            # /proc/mounts and avoid `assert not is_mountpoint` above.
+            def _():
+                if is_mountpoint(t.wc.mountpoint):
+                    subprocess.check_call(["fusermount", "-u", t.wc.mountpoint])
+            defer(_)
+
             eprint("\nC: wcfs.go does not exit")
             eprint("-> kill -QUIT wcfs.go ...\n")
             os.kill(t.wc._proc.pid, SIGQUIT)