Improve test suite

[jakob-keller]

Description of Change

Improve test suite:

• Remove @pytest.mark.asyncio decorators from test functions, in line with pytest-asyncio recommendation.
• reorganize test to match botocore
• inverse test selection
• fix and un-exclude tests in test_adaptive.py
• resolve warnings
• include test_eventstream.py from botocore

Cherry-picked from @thehesiod: f7a2f75

Assumptions

Replace this text with any assumptions made (if any)

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

Attention: Patch coverage is 99.51923% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.70%. Comparing base (e7d6ddf) to head (5aaa45a).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
tests/boto_tests/unit/test_eventstream.py	99.47%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
+ Coverage   86.16%   87.70%   +1.53%     
==========================================
  Files          64       65       +1     
  Lines        5986     5823     -163     
==========================================
- Hits         5158     5107      -51     
+ Misses        828      716     -112     

Flag	Coverage Δ
unittests	87.70% <99.51%> (+1.53%)	⬆️

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[thehesiod]

ty!