[test][utils] Add mock logging handler

This will allow to analyse the content of logs generated during
a test.

beat/cmdline/test/utils.py

@@ -29,6 +29,7 @@ Helper functions to run tests
 """
 
 import os
+import logging
 
 from beat.core.experiment import Experiment
 from beat.core.hash import toPath
@@ -53,3 +54,35 @@ def index_experiment_dbs(experiment_name):
                                       infos['set']),
                           suffix='.db')
         view.index(os.path.join(tmp_prefix, filename))
+
+
+# Based on https://stackoverflow.com/a/20553331/5843716
+class MockLoggingHandler(logging.Handler):
+    """Mock logging handler to check for expected logs.
+
+    Messages are available from an instance's ``messages`` dict, in order,
+    indexed by a lowercase log level string (e.g., 'debug', 'info', etc.).
+    """
+
+    def __init__(self, *args, **kwargs):
+        self.messages = {
+            'debug': [], 'info': [],
+            'warning': [], 'error': [],
+            'critical': [], 'extra': []
+        }
+        super(MockLoggingHandler, self).__init__(*args, **kwargs)
+
+    def emit(self, record):
+        "Store a message from ``record`` in the instance's ``messages`` dict."
+        try:
+            self.messages[record.levelname.lower()].append(record.getMessage())
+        except Exception:
+            self.handleError(record)
+
+    def reset(self):
+        self.acquire()
+        try:
+            for message_list in self.messages.values():
+                message_list.clear()
+        finally:
+            self.release()