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#1398 It's actually not possible to use a define inside a macro (see https://stackoverflow.com/questions/7246512/ifdef-inside-a-macro). I therefore had to duplicate the macro for the clang and not clang cases... (using a middleman macro didn't work as the warnings about macro recursion popped up - the very same for which I had to disambiguate clang and gcc in the first place in the macro...)

#1398 Fix bug in the macro (same test was run twice and another was therefore ignored) and add a Doxygen comment.

#1398 Attempt to fix compilation issue with static libraries: making test tools libraries build directly upon MoReFEM one led to multiple symbols. Changes work in macOS but remains to be checked on Linux ones.

#1398 Fix a test that didn't compile but wasn't visible in macOS (this test is neutralized as it requires std::variant not yet available there).

The reason for the change was partly bug #1363: tests were failing for macOS / shared library / release mode solely due to an internal error of Catch2.

Now we need to have Boost Test installed (as usual ThirdPartyCompilationLibrary facility provides it).

I also used the modifications to refresh the way tests were written:

- Fixture are now handled in a much better way; in peculiar some tests no longer required a fake model and its equally fake Lua file to proceed.
- The model tests have been refined to provide more information in case of failure (the complete path of the files that might not be equal are now directly given).
- Mpi fixture is now working completely: you may define several test suites in an executable without having it fail (previously it was the case only for the full-fledged model fixture).

This commit is unusually big for me, but that is due to the fact with macro everywhere in both Boost.Test and Catch2 I had rather not used them at the same time.

#1397 Delete temporary Yaml file and incorporate new macos jobs in the usual one. Tags have been added to 'specialize' mac runners in order to speed up jobs.

#1397 Deactivate tests in the dummy Yaml, and try to deactivate warnings for older versions of macOS.

#1396 Fix three warnings (one more elegantly than previously where the warning was neutralized locally).

#1396 Remove an unneeded include (which was moreover triggering a warning as it was in experimental directory).

#1360 - #1393 Add the guideline to use macOS machine (and provide Yaml to skip it if no credentials to Nostromo). Add the part explaining how to set up Yaml to use in gitlab.