add a pytest fixture for capturing logging stream

[Zsailer]

Fixes #585.

[codecov-commenter]

Codecov Report

Merging #588 (f3a5fa1) into master (ac93dd8) will decrease coverage by 0.15%.
The diff coverage is 91.30%.

❗ Current head f3a5fa1 differs from pull request most recent head c3a8d46. Consider uploading reports for the commit c3a8d46 to get more accurate results

@@            Coverage Diff             @@
##           master     #588      +/-   ##
==========================================
- Coverage   77.47%   77.32%   -0.16%     
==========================================
  Files         110      110              
  Lines       10212    10250      +38     
  Branches     1104     1257     +153     
==========================================
+ Hits         7912     7926      +14     
- Misses       1910     1928      +18     
- Partials      390      396       +6     

Impacted Files	Coverage Δ
jupyter_server/pytest_plugin.py	88.07% <77.77%> (-1.57%)	⬇️
jupyter_server/tests/services/kernels/test_api.py	97.02% <100.00%> (-1.95%)	⬇️
jupyter_server/tests/services/kernels/test_cull.py	100.00% <100.00%> (ø)
jupyter_server/tests/services/sessions/test_api.py	98.90% <100.00%> (+0.04%)	⬆️
jupyter_server/tests/test_terminal.py	98.75% <100.00%> (-0.05%)	⬇️
...yter_server/tests/services/kernelspecs/test_api.py	96.55% <0.00%> (-3.45%)	⬇️
jupyter_server/services/kernels/kernelmanager.py	79.79% <0.00%> (-2.06%)	⬇️
jupyter_server/services/kernels/handlers.py	60.92% <0.00%> (-1.55%)	⬇️
jupyter_server/utils.py	65.90% <0.00%> (-1.14%)	⬇️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ac93dd8...c3a8d46. Read the comment docs.

[blink1073]

Thanks!