Add Tests to Distribution

[afshin]

Moves tests to distributed package so they can be used downstream by consumers like conda-forge.

[blink1073]

Thanks!

[codecov-io]

Codecov Report

Merging #416 (5ba9f26) into master (7f63dde) will increase coverage by 7.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #416      +/-   ##
==========================================
+ Coverage   70.11%   77.61%   +7.50%     
==========================================
  Files          57      105      +48     
  Lines        6340     8932    +2592     
  Branches      850      955     +105     
==========================================
+ Hits         4445     6933    +2488     
- Misses       1591     1666      +75     
- Partials      304      333      +29     

Impacted Files	Coverage Δ
jupyter_server/tests/auth/test_login.py	92.10% <ø> (ø)
jupyter_server/tests/auth/test_security.py	100.00% <ø> (ø)
jupyter_server/tests/conftest.py	100.00% <ø> (ø)
jupyter_server/tests/extension/conftest.py	100.00% <ø> (ø)
..._server/tests/extension/mockextensions/__init__.py	100.00% <ø> (ø)
...pyter_server/tests/extension/mockextensions/app.py	88.46% <ø> (ø)
...ter_server/tests/extension/mockextensions/mock1.py	100.00% <ø> (ø)
...ter_server/tests/extension/mockextensions/mock2.py	100.00% <ø> (ø)
...ter_server/tests/extension/mockextensions/mock3.py	100.00% <ø> (ø)
...ver/tests/extension/mockextensions/mockext_both.py	75.00% <ø> (ø)
... and 88 more

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