One server for unit tests

[ca-johnson]

• Modify scope of fixture to create the server one time and re-use it for each test
• p4trust is a property of client rather than server - move that file into the unit tests where it is relevant, in tmpdir for client

Improves time to run unit tests locally:

Initial:
$ python -m pytest python/test_perforce.py    
=== 19 passed in 25.80s ===

With change:
$ python -m pytest python/test_perforce.py    
=== 19 passed in 7.16s ===

With pytest-xdist:
$ python -m pytest python/test_perforce.py --numprocesses=auto  
=== 19 passed in 3.98s ===

[improbable-prow-robot]

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[ca-johnson]

Noticed that when a unit test fails (with this PR), there is a lot of spam due to attempts to write to a log that is already closed. Will try to resolve this prior to merge

[ca-johnson]

I was able to narrow the above down to the use of capsys in test_server_fixture unit test.

We could remove this, but it makes it harder to see what port the server is running on when trying to modify the server fixture.

Its possible that this is fixed in a newer version of pytest, which dependabot has a PR open for: #197

Its not a huge deal, but probably worth looking into to remove the distracting logging errors vs failure

Ticketed at #204

Going to merge this for now, since it makes it noticeably faster to iterate on resolving this.