Clear env before test cases requiring empty env

[tkonopka]

What does this PR do?

Fixes #12551

An existing test case currently runs well with python -m pytest tests/ but fails with make test because the Makefile changes an env variable. The fix updates the test case so that it sees the same env state under the two scenarios.

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[akihironitta]

Thank you @tkonopka for your quick action!

There seem to be other occurrences which I believe need to be updated in the tests. Could you also update the following lines?

$ grep -rnI "os.environ, {}" tests
tests/loggers/test_tensorboard.py:126:@mock.patch.dict(os.environ, {})
tests/callbacks/test_gpu_stats_monitor.py:145:@mock.patch.dict(os.environ, {})
tests/plugins/environments/test_lsf_environment.py:104:    with mock.patch.dict(os.environ, {}):
tests/plugins/environments/test_lightning_environment.py:22:@mock.patch.dict(os.environ, {})
tests/plugins/environments/test_lightning_environment.py:68:@mock.patch.dict(os.environ, {})
tests/plugins/environments/test_slurm_environment.py:23:@mock.patch.dict(os.environ, {})
tests/plugins/environments/test_slurm_environment.py:95:    with mock.patch.dict(os.environ, {}):
tests/plugins/environments/test_torchelastic_environment.py:24:@mock.patch.dict(os.environ, {})
tests/plugins/environments/test_torchelastic_environment.py:77:    with mock.patch.dict(os.environ, {}):
tests/plugins/environments/test_torchelastic_environment.py:95:    with mock.patch.dict(os.environ, {}):
tests/plugins/environments/test_kubeflow_environment.py:23:@mock.patch.dict(os.environ, {})

[tkonopka]

Done the additional changes. This will help the suite run in environments that pre-set variables. Thanks.

Earlier, some of the automatic checks seemed stuck (4 expected, and 1 pending checks). I was waiting on those before marking the PR as 'ready', but happy to follow your lead @akihironitta.

[akihironitta]

@tkonopka Thank you for applying the changes! LGTM!