Add more pointcloud extension tests

[gadomski]

Related Issue(s): #248

Description: Increase coverage for pointcloud extension tests. Doesn't go 100% b/c I don't like testing __repr__ and there's some property getter paths that don't seem worth exploring because they seem like they're already protected in one of the superclasses.

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #352 (3e2d51c) into main (016e56c) will increase coverage by 0.84%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #352      +/-   ##
==========================================
+ Coverage   88.59%   89.43%   +0.84%     
==========================================
  Files          36       36              
  Lines        4629     4629              
==========================================
+ Hits         4101     4140      +39     
+ Misses        528      489      -39     

Impacted Files	Coverage Δ
pystac/extensions/pointcloud.py	96.61% <0.00%> (+16.52%)	⬆️

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[lossyrob]

LGTM - @duckontheweb I'll leave it to you to merge this, as it will effect the total code coverage in a potentially disruptive way if you are tracking it as part of #331

[duckontheweb]

LGTM - @duckontheweb I'll leave it to you to merge this, as it will effect the total code coverage in a potentially disruptive way if you are tracking it as part of #331

Sounds good. I'm going to go ahead and merge since the changes are all positive and pretty isolated, but thanks for checking.