added unusedFunction self check to CI / cleanups

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I filed https://trac.cppcheck.net/ticket/10660 and https://trac.cppcheck.net/ticket/10661 for the false positives.

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I filed https://trac.cppcheck.net/ticket/10667 for the preprocessorErrorDirective.

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The environment variable is a temporary workaround until we properly disable the ValueFlow based on the configuration.

I will add more tickets for some of the TODOs later on.

I wanted to combine this with the other self check run and parallelize them but the unusedFunction one is very fast so that might not be necessary. It's separate for now but I will bundle it with the other one at a later point.

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I added https://trac.cppcheck.net/ticket/10697 for the unmatchedSupression TODO.

I would have preferred to have less suppressions in the code but with this we can run it in the CI and will prevent us from accumulating more warnings with future changes. Further cleanups can always be done in future commits.

The disabled run will only make us miss unused function within the test code which is fine as it will still catch all the unused application code.

There's a false negative detected by GCC and Clang as a compiler warning which I will add a ticket for soon.

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@danmar This is ready for review.