Document how to see logger output when running the twisted tests (#10148)

CONTRIBUTING.md

@@ -173,12 +173,19 @@ source ./env/bin/activate
 trial tests.rest.admin.test_room tests.handlers.test_admin.ExfiltrateData.test_invite
 ```
 
-If your tests fail, you may wish to look at the logs:
+If your tests fail, you may wish to look at the logs (the default log level is `ERROR`):
 
 ```sh
 less _trial_temp/test.log
 ```
 
+To increase the log level for the tests, set `SYNAPSE_TEST_LOG_LEVEL`:
+
+```sh
+SYNAPSE_TEST_LOG_LEVEL=DEBUG trial tests
+```
+
+
 ## Run the integration tests.
 
 The integration tests are a more comprehensive suite of tests. They

README.rst

@@ -293,18 +293,6 @@ try installing the failing modules individually::
 
     pip install -e "module-name"
 
-Once this is done, you may wish to run Synapse's unit tests to
-check that everything is installed correctly::
-
-    python -m twisted.trial tests
-
-This should end with a 'PASSED' result (note that exact numbers will
-differ)::
-
-    Ran 1337 tests in 716.064s
-
-    PASSED (skips=15, successes=1322)
-
 We recommend using the demo which starts 3 federated instances running on ports `8080` - `8082`
 
     ./demo/start.sh
@@ -324,6 +312,23 @@ If you just want to start a single instance of the app and run it directly::
     python -m synapse.app.homeserver --config-path homeserver.yaml
 
 
+Running the unit tests
+======================
+
+After getting up and running, you may wish to run Synapse's unit tests to
+check that everything is installed correctly::
+
+    trial tests
+
+This should end with a 'PASSED' result (note that exact numbers will
+differ)::
+
+    Ran 1337 tests in 716.064s
+
+    PASSED (skips=15, successes=1322)
+
+For more tips on running the unit tests, like running a specific test or
+to see the logging output, see the `CONTRIBUTING doc <CONTRIBUTING.md#run-the-unit-tests>`_.
 
 
 Running the Integration Tests

changelog.d/10148.misc

+Document `SYNAPSE_TEST_LOG_LEVEL` to see the logger output when running tests.