coverage: Omit some test runner files from coverage.

These files are part of the test runner and can't realisitically have
test coverage due to being imported before coverage in the setup
sequence.

tools/coveragerc

@@ -20,3 +20,7 @@ omit =
     */zulip-venv-cache/*
     */migrations/*
     */management/commands/*
+    tools/test-backend
+    zerver/lib/test_fixtures.py
+    zerver/lib/test_runner.py
+    scripts/lib/zulip_tools.py