Move test_config to align with other packages

[mjcarroll]

🦟 Bug fix

Summary

This moves the generated test_config.h to not be in test, which brings it in line with the rest of the ignition packages.

It also rearranges a bit of the gtest cmake logic so that the targets/includes are more explicit.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #246 (20d1df6) into main (bd50422) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

❗ Current head 20d1df6 differs from pull request most recent head 2cdad7f. Consider uploading reports for the commit 2cdad7f to get more accurate results

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
- Coverage   75.57%   75.56%   -0.02%     
==========================================
  Files          20       20              
  Lines        2850     2848       -2     
==========================================
- Hits         2154     2152       -2     
  Misses        696      696              

Impacted Files	Coverage Δ
include/ignition/fuel_tools/Result.hh	100.00% <ø> (ø)
src/FuelClient.cc	67.77% <0.00%> (ø)
src/RestClient.cc	67.42% <100.00%> (-0.49%)	⬇️

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