Cleanup asdf._tests._helpers

[braingram]

Description

This PR cleans up asdf._tests._helpers removing all but assert_tree_match. This PR removes:

• get_test_data_path replaced with a test fixture using importlib.resources (data for commands tests were moved to _tests.data to simplify paths)
• assert_roundtrip_tree replaced with asdf.testing.helpers.assert_roundtrip_object and more explicit assert statements to compare the feature being tested in any given test
• yaml_to_asdf replaced with asdf.testing.helpers.yaml_to_asdf
• get_file_sizes replaced with uses of pathlib.Path and calls to stat().st_size
• display_warnings unused, removed
• assert_no_warnings all uses removed, tests already run with a filter that turns warnings into errors, a pytest configuration was added to _tests/conftest to make sure that this filter applies to all runs (even through through pytest --pyargs asdf as is run during the Build CI test)
• _assert_extension_type_correctness unused, removed

Although not removed assert_tree_match was simplified (removing unused arguments and other unused code).

assert_roundtrip_tree performed a number of tests for round tripping (that are not all performed by testing.helpers.roundtrip_object):

• read/write to BytesIO, tested with assert_roundtrip_object
• read/write to a real file, low-level file access modes (like real files and BytesIO) are tests in test_generic_io.py
• read from an http range server, http access is tested in test_generic_io.py and via fsspec in test_asdf.py
• read with lazy loading an no memmapping, low-level file access is tests in test_generic_io.py and memmapping is tested in test_array_blocks.py
• read with memmapping memmapping is tested in test_array_blocks.py

The simplification of assert_tree_match will further break the test_reference_files.py tests that have not been running since Feb 2022. When run manually (by checking out asdf-standard as if it were still a submodule) these fail without this PR and likely these should be updated and possibly moved to the asdf-standard test suite (which is run as part of the asdf CI). There is already an open issue for these tests: #1522

Removal of assert_roundtrip_tree allowed for the removal of the bundled extern.RangeHTTPServer (removed in this PR).

Fixes #798

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[zacharyburnett]

this looks good to me and simplifies the test fixtures; can it be rebased or is this branch too far gone to be worth it?

[braingram]

Thanks for the review! I will (attempt to) resolve the conflicts 😬