fsinfo: change an if: to an assert

fsinfo was already at 100% lines coverage, but this allows it to reach
100% branch coverage as well.

src/cockpit/channels/filesystem.py

@@ -493,8 +493,9 @@ def do_identity_changed(self, fd: 'Handle | None', err: 'int | None') -> None:
             self.report_error(err)
 
     def do_close(self) -> None:
-        if self.path_watch is not None:
-            self.path_watch.close()
+        # non-watch channels close immediately — if we get this, we're watching
+        assert self.path_watch is not None
+        self.path_watch.close()
         self.close()
 
     def do_open(self, options: JsonObject) -> None: