coverage: Exclude api_test_helpers.py from backend test coverage.

These files are tested by a different test suite, and in practice have
100% coverage; no need to double-measure it.

tools/coveragerc

@@ -21,6 +21,8 @@ omit =
     tools/test-backend
     zerver/lib/test_fixtures.py
     zerver/lib/test_runner.py
+    # For API tests
+    zerver/lib/api_test_helpers.py
     # Part of provisioning/populate_db
     zerver/lib/generate_test_data.py
     # Excluded because its coverage state is flaky.