Fix unit tests that were silently skipped or crashed (depending on the compiler)

[grembo]

Make section names unique in loops, as catch doesn't support duplicate sections, see also catchorg/Catch2#816 (comment)

As a result, when built with gcc, loop iterations were skipped. When
built with clang, the test aborted with an assertion in catch.hpp
line 6222.

This also addresses the issues discussed here:
#1032 (comment)

and here:
catchorg/Catch2#1241

As some of the unit tests which never ran before failed now, I added an exclude list for one of the tests (this is due to nlohmann-json ordering dictionaries by key, while the output files it compares against kept the key order of the json files).

Also, the way the tests were written, they would now take a long time, therefore I moved parsing the source into individual sections (more code, but 99% less parsing operations for the same tests).

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[coveralls]

Coverage remained the same at 100.0% when pulling 05b27e8 on grembo:develop into 3760a38 on nlohmann:develop.

[nlohmann]

Thanks a lot for the PR. I shall review it shortly.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks a lot!