Remove dependency on pytest-subtests.

[bdice]

Description

This PR changes a small number of tests that relied on the pytest-subtests plugin.

Motivation and Context

The pytest-subtests plugin was causing some trouble because upgrades to pytest broke compatibility with pytest-subtests on two separate occasions:

1. 0.3.1 to 0.3.2 breakage is documented here: Bump pytest from 5.4.3 to 6.0.1 #366, Bump pytest from 5.4.3 to 6.0.1 #366 (comment).
2. 0.3.0 to 0.3.1 breakage is not documented but I remember running into it on my own, and found a comment to corroborate this on Slack.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[codecov]

Codecov Report

Merging #379 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
- Coverage   76.49%   76.46%   -0.03%     
==========================================
  Files          45       45              
  Lines        7130     7130              
==========================================
- Hits         5454     5452       -2     
- Misses       1676     1678       +2     

Impacted Files	Coverage Δ
signac/contrib/project.py	91.32% <0.00%> (-0.28%)	⬇️

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[bdice]

@klywang Can you please review?

[klywang]

Everything looks good!