test-backend: Enable --ban-console-output and document.

docs/testing/testing-with-django.md

@@ -436,6 +436,13 @@ code and edge cases.  It will generate a nice HTML report that you can
 view right from your browser (the tool prints the URL where the report
 is exposed in your development environment).
 
+- **Console output** A properly written test should print nothing to
+the console; use `with self.assertLogs` to capture and verify any
+logging output.  Note that we reconfigure various loggers in
+`zproject/test_extra_settings.py` where the output is unlikely to be
+interesting when running our test suite.  `test-backend
+--ban-console-output` checks for stray print statements.
+
 Note that `test-backend --coverage` will assert that
 various specific files in the project have 100% test coverage and
 throw an error if their coverage has fallen.  One of our project goals

tools/ci/backend

@@ -12,7 +12,7 @@ set -x
 # docker setup means the auto-detection logic sees the ~36 processes
 # the Docker host has, not the ~2 processes of resources we're
 # allocated.
-./tools/test-backend --coverage --include-webhooks --no-cov-cleanup --parallel=6
+./tools/test-backend --coverage --include-webhooks --no-cov-cleanup --ban-console-output --parallel=6
 
 # We run mypy after the backend tests so we get output from the
 # backend tests, which tend to uncover more serious problems, first.

tools/test-all

@@ -49,7 +49,7 @@ run ./tools/clean-repo
 # ci/backend
 run ./tools/lint --groups=backend $FORCEARG
 run ./tools/test-tools
-run ./tools/test-backend --include-webhooks $FORCEARG
+run ./tools/test-backend --include-webhooks --ban-console-output $FORCEARG
 run ./tools/test-migrations
 # Not running SVG optimizing since it's low-churn
 # run ./tools/setup/optimize-svg