Fix running tests in environment with FORCE_COLOR or NO_COLOR set

[mgorny]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

Ensure to unset FORCE_COLOR and NO_COLOR environment variables within the scope of individual tests, in order to fix test failures when these variables are set in the environment where tests are run, e.g. via:

NO_COLOR=1 tox

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (aabfd16) 98.30% compared to head (7d79acb) 98.28%.
Report is 41 commits behind head on master.

Files	Patch %	Lines
rich/text.py	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3226      +/-   ##
==========================================
- Coverage   98.30%   98.28%   -0.03%     
==========================================
  Files          74       74              
  Lines        8038     8049      +11     
==========================================
+ Hits         7902     7911       +9     
- Misses        136      138       +2     

Flag	Coverage Δ
unittests	98.28% <98.03%> (-0.03%)	⬇️

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[mgorny]

Thanks!