Address warnings produced by tests

[kevin-bates]

This pull request takes a look at removing/ignoring a majority of the warnings that are produced by the server tests (via make test-server). Many of these warnings are ignored as they stem from upstream modules. That said, there are a number that have been resolved via code changes (primarily in tests) where things like file descriptors were not closed, etc.

We should strive to get to a place where all warnings (except those explicitly handled) are treated as errors. This will enable their capture in a timely manner so things like DeprecationWarning can be handled and adjusted (if necessary) or added to the list of ignored warnings (if appropriate).

This PR adds a number of warnings that are now ignored where, like caps on dependencies, a comment is provided indicating why the warning is ignored and/or additional context about the warning. At this point, the error filter is disabled. We could also consider ignoring these warnings using decorators in the test files themselves.

It looks like the number of warnings varies based on which version of Python is in use, but I'm seeing the warnings decrease from thousands of warnings to these 15:

================================================================================= warnings summary =================================================================================
elyra/tests/pipeline/test_pipeline_definition.py: 2 warnings
elyra/tests/pipeline/test_pipeline_parser.py: 1 warning
elyra/tests/pipeline/test_validation.py: 1 warning
elyra/tests/pipeline/airflow/test_component_parser_airflow.py: 9 warnings
elyra/tests/pipeline/airflow/test_processor_airflow.py: 2 warnings
  <unknown>:26: DeprecationWarning: invalid escape sequence '\_'

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================================================================================ slowest durations =================================================================================

(1714 durations < 60s hidden.  Use -vv to show these durations.)
============================================================= 570 passed, 2 skipped, 15 warnings in 347.53s (0:05:47) ==============================================================

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[kevin-bates]

Moving to draft to try to address remaining warnings.

[kevin-bates]

The last 15 warnings will go away once this PR is merged. The reason is that one of the test resources is referenced within the main GitHub branch within the tests, so those changes won't be available until after the merge. (I temporarily pointed at my branch to confirm.)

I also fixed the test-artifact "leak" where the test_processor_kfp.py tests were producing these files in the dev area:

	elyra/tests/pipeline/resources/test_pipelines/kfp/kfp-one-node-generic-elyra-properties.yaml
	elyra/tests/pipeline/resources/test_pipelines/kfp/kfp-one-node-generic.yaml

This ultimately interferes with the dev's ability to check in changes w/o first removing those files or being explicit on each commit.

And removed the use of the requests.session() function, which was deprecated, and replaced it with requests.sessions.Session().

[ptitzler]

LGTM!