Cleanup of tests, and removal of _assert_raises

[jakkdl]

Improvements to tests, including removal of _assert_raises, pulled out from #2886

[codecov]

Codecov Report

Merging #2890 (b9acf48) into master (9c496fa) will decrease coverage by 0.01%.
Report is 3 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2890      +/-   ##
==========================================
- Coverage   99.54%   99.54%   -0.01%     
==========================================
  Files         115      115              
  Lines       17660    17623      -37     
  Branches     3165     3153      -12     
==========================================
- Hits        17580    17543      -37     
  Misses         52       52              
  Partials       28       28              

Files	Coverage Δ
src/trio/_core/_tests/test_run.py	100.00% <100.00%> (ø)
src/trio/_tests/test_ssl.py	99.85% <100.00%> (-0.01%)	⬇️
src/trio/_tests/test_util.py	100.00% <100.00%> (ø)
src/trio/testing/_check_streams.py	100.00% <ø> (ø)

... and 4 files with indirect coverage changes

[jakkdl]

The coverage change is from mypy stuff in test_exports, idk why it's popping up here but should probably get some pragma: no cover in another PR.

[A5rocks]

This looks good