test: router mocker E2E tests to run serially for stability (#6018)

Signed-off-by: Keiven Chang <keivenchang@users.noreply.github.com>
Co-authored-by: Keiven Chang <keivenchang@users.noreply.github.com>

tests/router/test_router_e2e_with_mockers.py

@@ -43,7 +43,6 @@ pytestmark = [
     pytest.mark.pre_merge,
     pytest.mark.gpu_0,
     pytest.mark.integration,
-    pytest.mark.parallel,
     pytest.mark.model(MODEL_NAME),
 ]
 NUM_MOCKERS = 2