Use fewer contours in testing roi_objects

[JorgeGtz]

Describe your changes
The files used in the tests (roi_objects and object_composition) before contained ~1800 contours, which can be computationally heavy.
This PR reduces the numbers of contours used.

Type of update
Is this a:

• Reduction in test time

Associated issues
NA

Additional context
The tests take a long time to complete.

[codecov]

Codecov Report

Merging #844 (71e918a) into master (b51b545) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            master      #844    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          152       142    -10     
  Lines         6919      6223   -696     
==========================================
- Hits          6919      6223   -696     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
plantcv/__init__.py	100.00% <0.00%> (ø)
plantcv/plantcv/_debug.py	100.00% <0.00%> (ø)
plantcv/plantcv/classes.py	100.00% <0.00%> (ø)
plantcv/plantcv/plot_image.py	100.00% <0.00%> (ø)
plantcv/plantcv/print_image.py	100.00% <0.00%> (ø)
plantcv/plantcv/photosynthesis/__init__.py	100.00% <0.00%> (ø)
...lantcv/plantcv/photosynthesis/read_cropreporter.py	100.00% <0.00%> (ø)
plantcv/plantcv/homology/constellaqc.py
plantcv/plantcv/homology/starscape.py
plantcv/plantcv/_show_dataarray.py
... and 7 more

[nfahlgren]

Nice!