Test improvements - part 1

[soapy1]

partially addresses https://github.com/conda-incubator/conda-store/issues/914

Description

This PR makes a few improvements to the tests for conda-store-server:

• restructures the conda-store-server/test/ directory to more closely match the directory structure of the conda-store-server app
• fixes the utc datetime.datetime.utcfromtimestamp() deprecation warning
  • cancel build actions fail as a result of fixing this warning. Presumably because a dependency that conda-store relies on does not support this
• adds a pytest marker for slow tests to allow developers to not run slow tests locally (this is just a dev experience improvement, all tests should continue to be run in CI). In order to use this, try python -m pytest -m "not extended_prefix and not user_journey and not long_running_test" tests/

As a result of the updates to the fixtures, running tests should be much faster. Before this change, running locally

$ python -m pytest -m "not extended_prefix and not user_journey and not long_running_test" --durations=0 tests/
. . .
======================== 1 failed, 225 passed, 9 deselected, 2 xfailed, 1338 warnings in 1201.96s (0:20:01) ==============================

After,

==== 1 failed, 198 passed, 38 deselected, 816 warnings in 389.80s (0:06:29) =====

Note, this change is mostly because of marking some particularly long running tests.

Pull request checklist

• [x] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

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