cloud_storage/ducktape: Wait for chunk obs. to stop

Tests which use chunk observer should wait for it to stop after the stop
command is issued, to avoid the underlying set changing while it is
being read from.

(cherry picked from commit 524a230)

tests/rptest/tests/cloud_storage_chunk_read_path_test.py

@@ -346,6 +346,9 @@ def test_read_when_cache_smaller_than_segment_size(self):
 
         self._consume_baseline(timeout=180, max_msgs=read_count)
         observe_cache_dir.stop()
+        observe_cache_dir.join(timeout=10)
+        assert not observe_cache_dir.is_alive(
+        ), 'cache observer is unexpectedly alive'
 
         self._assert_not_in_cache(fr'.*kafka/{self.topic}/.*\.log\.[0-9]+$')
         self._assert_in_cache(f'.*kafka/{self.topic}/.*_chunks/[0-9]+')