Enable configuring test log level via env var (#4506)

I got fed up with always adding '@unittest.DEBUG' every time I needed to debug a test.

tests/test_utils/logging_setup.py

@@ -0,0 +1,54 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+import os
+
+import twisted.logger
+
+from synapse.util.logcontext import LoggingContextFilter
+
+
+class ToTwistedHandler(logging.Handler):
+    """logging handler which sends the logs to the twisted log"""
+    tx_log = twisted.logger.Logger()
+
+    def emit(self, record):
+        log_entry = self.format(record)
+        log_level = record.levelname.lower().replace('warning', 'warn')
+        self.tx_log.emit(
+            twisted.logger.LogLevel.levelWithName(log_level),
+            log_entry.replace("{", r"(").replace("}", r")"),
+        )
+
+
+def setup_logging():
+    """Configure the python logging appropriately for the tests.
+
+    (Logs will end up in _trial_temp.)
+    """
+    root_logger = logging.getLogger()
+
+    log_format = (
+        "%(asctime)s - %(name)s - %(lineno)d - %(levelname)s - %(request)s - %(message)s"
+    )
+
+    handler = ToTwistedHandler()
+    formatter = logging.Formatter(log_format)
+    handler.setFormatter(formatter)
+    handler.addFilter(LoggingContextFilter(request=""))
+    root_logger.addHandler(handler)
+
+    log_level = os.environ.get("SYNAPSE_TEST_LOG_LEVEL", "ERROR")
+    root_logger.setLevel(log_level)