Remove signac.testing

[vyasr]

Description

Remove the signac.testing module.

Motivation and Context

Resolves #709.

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.
• 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.

[codecov]

Codecov Report

Merging #863 (77de33e) into master (986446b) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #863      +/-   ##
==========================================
- Coverage   86.67%   86.63%   -0.05%     
==========================================
  Files          49       48       -1     
  Lines        4585     4570      -15     
  Branches      908      901       -7     
==========================================
- Hits         3974     3959      -15     
  Misses        430      430              
  Partials      181      181              

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