Move plugins to TestPlugins dir

[jbeirer]

This pull request:

• moves the ATLAS FCAL geo plugin out of the FCS main code into a new TestPlugins/ATLAS directory
• moves the thirdparty/ATLASMagneticFieldMapPlugin into the TestPlugins/ATLAS directory

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 36.49%. Comparing base (f21a700) to head (bed1af0). Report is 1 commits behind head on main.

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
.../ATLASMagneticFieldMapPlugin/src/AtlasFieldSvc.cxx	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #42 +/- ## ======================================= Coverage 36.49% 36.49% ======================================= Files 211 211 Lines 9478 9478 Branches 1334 1334 ======================================= Hits 3459 3459 Misses 6019 6019 ```

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