Install pytest in e2e_browser workflow

Matches the explicit pytest install in test_dev / test_stable workflows.
Neither requirements.txt nor dev_requirements.txt lists pytest, so the
e2e job hit ModuleNotFoundError when invoking python -m pytest.

Author: JE-Chen

.github/workflows/e2e_browser.yml

@@ -46,6 +46,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip wheel
           pip install -r dev_requirements.txt
+          pip install pytest
 
       - name: Wait for Grid to be ready
         run: |