[All] Fix subplugin regression tests

[bakpaul]

[ci-depends-on https://github.com/sofa-framework/Regression/pull/56]
[ci-depends-on https://github.com/sofa-framework/BeamAdapter/pull/125]

This PR is based on PR #56 of Regression.

This PR fixes the problem of running regression tests in plugins when their references are inside of the plugin and not Regression (e.g. BeamAdapter)

The idea is to have a stand alone *.regression.tests file that also includes the relative path of the reference folder. This is meant to be placed at the top of the reference file in a relative manne. This allows two things :

1. Having a stand alone file ease the process of finding the reference folder for any new comer
2. Having a stand alone file ease the process of automatizing the retrieval of new regression test along with their reference folder without the need of modifying anything in the CI script.

Tested on my side : only two remaining regression test failing from SofaSPH

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[bakpaul]

[ci-build][with-all-tests][force-full-build]

[sofabot]

[ci-depends-on] detected during build #2.

To unlock the merge button, you must

• Merge or close Fix subplugin regression tests Regression#56
  For this build, Regression/ExternalProjectConfig.cmake.in will be edited by builders with
  GIT_REPOSITORY https://github.com/sofa-framework/Regression
  GIT_TAG 4d160b90f2e54fc6bfa25e260f3c25dd1af81500

[bakpaul]

[ci-build][with-all-tests][force-full-build]

[sofabot]

[ci-depends-on] detected during build #3.

To unlock the merge button, you must

• Merge or close Fix subplugin regression tests Regression#56
  For this build, Regression/ExternalProjectConfig.cmake.in will be edited by builders with
  GIT_REPOSITORY https://github.com/sofa-framework/Regression
  GIT_TAG 4d160b90f2e54fc6bfa25e260f3c25dd1af81500
• Merge or close Add relative path to references files into regression-tests files BeamAdapter#125
  For this build, BeamAdapter/ExternalProjectConfig.cmake.in will be edited by builders with
  GIT_REPOSITORY https://github.com/sofa-framework/BeamAdapter
  GIT_TAG 3295e868abc0f700b24df81c9fa4a50fdfd455ee

[hugtalbot]

Note: we only have regressions for BeamAdapter, Multithreading and SphFluid

[bakpaul]

Create Issue to rethink the way it is done: maybe centralize the regression-tests files in Regression plugin. This would require multiple changes like being able to find the path of all plugins to find the scenes and the regression files when they are inside of the plugin instead of Regression.

--> #4424