Restore pytest plugin from pytest-jupyter

[kevin-bates]

In today's Jupyter Server meeting, we discussed deprecating the pytest-jupyter library and moving (back) to a native pytest_plugin in this package (how we originally had it).

The pytest-jupyter library introduced extra complexity, requiring us to coordinate releases of pytest-jupyter and jupyter_server anytime there was a change in jupyter_server.

Here, the pytest_plugin is a submodule in jupyter_server. To use this plugin in a given test suite, add jupyter_server.pytest_plugin to the pytest_plugin list of the root conftest.py file, e.g.:

# conftest.py file
pytest_plugin = [
    "jupyter_server.pytest_plugin",
    ...
]

This PR includes some fixtures that we should eventually move to the jupyter_core package as a jupyter_core.pytest_plugin module. We can do this at a later time. Jupyter Server can remove those fixtures and depend on jupyter_core's plugin without changing API or disrupting downstream projects.

[codecov-io]

Codecov Report

Merging #360 (1858bb5) into master (ca492c1) will increase coverage by 0.85%.
The diff coverage is 93.12%.

@@            Coverage Diff             @@
##           master     #360      +/-   ##
==========================================
+ Coverage   67.37%   68.23%   +0.85%     
==========================================
  Files          55       56       +1     
  Lines        5361     5550     +189     
  Branches      696      705       +9     
==========================================
+ Hits         3612     3787     +175     
- Misses       1545     1556      +11     
- Partials      204      207       +3     

Impacted Files	Coverage Δ
jupyter_server/pytest_plugin.py	93.12% <93.12%> (ø)
jupyter_server/serverapp.py	62.00% <0.00%> (-0.13%)	⬇️

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[Zsailer]

LGTM!