Remove duplicated functionality from test_config

[mjcarroll]

• Use gz::utils::env family of functions rather than the OS equivalents (which are also missing on Windows)
• Use gz::utils disable macros rather than custom ones.
• Remove std::filesystem logic, as we are only supporting c++17 and above here.

[codecov[bot]]

Codecov Report

Merging #457 (bb52bd6) into gz-transport12 (e21d910) will not change coverage. The diff coverage is n/a.

:exclamation: Current head bb52bd6 differs from pull request most recent head 798e7f6. Consider uploading reports for the commit 798e7f6 to get more accurate results

@@               Coverage Diff               @@
##           gz-transport12     #457   +/-   ##
===============================================
  Coverage           87.29%   87.29%           
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  Files                  60       60           
  Lines                5211     5211           
===============================================
  Hits                 4549     4549           
  Misses                662      662