Fix flaky clean_removed test (elastic#8092)

This is a potential fix for the flaky clean_removed test. The problem in the test seems to be that sometimes not all states are cleaned up yet. This is changing it by waiting for all pending cleanups to happen.

Closes elastic#7690

filebeat/tests/system/test_registrar.py

@@ -895,6 +895,9 @@ def test_clean_removed(self):
             lambda: self.output_has(lines=3),
             max_timeout=10)
 
+        # Make sure all states are cleaned up
+        self.wait_until(lambda: self.log_contains("Before: 1, After: 1, Pending: 0"))
+
         filebeat.check_kill_and_wait()
 
         # Check that the first to files were removed from the registry