Augment test to also cover `GET /v2/stored flows` installed state

[viniarck]

This complements PR #175

I augmented test_005_install_flow to also cover GET /v2/stored flows installed state.

Local tests

I ran it three times:

+ for i in {1..3}
+ echo 'iteration 3'
iteration 3
+ python3 -m pytest tests/ --timeout=300 -k 'test_e2e_20_flow_manager and test_005_install_flow' -vv
============================= test session starts ==============================
platform linux -- Python 3.9.2, pytest-6.2.5, py-1.11.0, pluggy-1.0.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /tests
plugins: timeout-2.0.2
timeout: 300.0s
timeout method: signal
timeout func_only: False
collecting ... collected 200 items / 199 deselected / 1 selected

tests/test_e2e_20_flow_manager.py::TestE2EFlowManager::test_005_install_flow PASSED [100%]

=============================== warnings summary ===============================
test_e2e_20_flow_manager.py: 17 warnings
  /usr/lib/python3/dist-packages/mininet/node.py:1121: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
    return ( StrictVersion( cls.OVSVersion ) <

test_e2e_20_flow_manager.py: 17 warnings
  /usr/lib/python3/dist-packages/mininet/node.py:1122: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
    StrictVersion( '1.10' ) )

-- Docs: https://docs.pytest.org/en/stable/warnings.html
------------------------------- start/stop times -------------------------------
========== 1 passed, 199 deselected, 34 warnings in 106.73s (0:01:46) ==========

[viniarck]

Appreciated your review, Italo.