caddy-frontend: Disable ATS cache promise plugin

ATS cache fillup is uncontrollable during test run.

software/caddy-frontend/test/test.py

@@ -359,6 +359,8 @@ class TestDataMixin(object):
     ignored_plugin_list = [
       '__init__.py',  # that's not a plugin
       'monitor-http-frontend.py',  # can't check w/o functioning frontend
+      # ATS cache fillup can't be really controlled during test run
+      'trafficserver-cache-availability.py',
     ]
     runpromise_bin = os.path.join(
       self.software_path, 'bin', 'monitor.runpromise')