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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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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[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.