Garden test cleanup

[mjcarroll]

Split from #513 for easier review.

This cleans up physics tests in a few ways:

• Consolidate TestUtilities.hh and test/Utils.hh
• Move test headers to test/include/test
• Move mock headers to test/include/mock
• Create an INTERFACE library for test/mock headers that sets common compiler definitions as well as common libraries to link against
• Remove all instances of hardcoded world names or resource names and instead use constants.

[mjcarroll]

A question here (maybe for @scpeters @ahcorde). There are worlds/sdf files that are spread out among the various implementations. Can these be deduplicated/consolidated?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5367620) 78.64% compared to head (e84feed) 78.47%.

❗ Current head e84feed differs from pull request most recent head 1c806fd. Consider uploading reports for the commit 1c806fd to get more accurate results

Additional details and impacted files

@@               Coverage Diff               @@
##           gz-physics6     #587      +/-   ##
===============================================
- Coverage        78.64%   78.47%   -0.17%     
===============================================
  Files              141      139       -2     
  Lines             7713     7611     -102     
===============================================
- Hits              6066     5973      -93     
+ Misses            1647     1638       -9     

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[ahcorde]

Nice clean up! Just some minor comments

[scpeters]

I'm not finished reviewing this, but here's a batch of comments

[mjcarroll]

@scpeters I have removed anything that was duplicated (or closely duplicated). This required some small changes in the tpe tests to match the common world files.