Use more features of unittest

[cbkerr]

Description

• use assertTrue as recommended in examples (may switch to more specific assertIn for some)
• use subTest for better messages on test failures

Motivation and Context

Trying to make it easier to add tests to our PRs #155, #152, #132 .

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• The package documentation and framework documentation in signac-docs are up to date with these changes.
• I have updated the changelog and added any related issue and pull request numbers for future reference.

[cbkerr]

I'd like help refactoring this.

[codecov]

Codecov Report

Merging #156 (abebc3d) into master (1315fbd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #156   +/-   ##
=======================================
  Coverage   72.40%   72.40%           
=======================================
  Files          20       20           
  Lines         750      750           
=======================================
  Hits          543      543           
  Misses        207      207           

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[bdice]

Rather than doubling down on unittest paradigms here, I'd recommend porting our tests to use pytest. pytest has helpful error messages for plain assert statements.

[bdice]

@cbkerr I'm going to close this PR. If we were to apply these changes, it would actually move us away from pytest API design / paradigms, which we want to embrace. I haven't had time to migrate the testing suite fully to pytest, but it is an eventual task that I plan to take on after cleaning up some of the backlog of signac 2.0 and signac-dashboard 0.4.0 work. I opened #164 to track this.