Fix test cache dir

[nfahlgren]

Describe your changes
This PR sets the pcv.params.debug_outdir to the test cache tmp directory for the test test_segment_image_series to prevent test files from being saved to the source directory. Minor code format changes are also included

Type of update
Is this a: Bug fix

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

Merging #957 (0929162) into release-4.0 (fe40c16) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           release-4.0      #957   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              161       161           
  Lines             6859      6859           
=============================================
  Hits              6859      6859           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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