some missingInclude/missingIncludeSystem fixes and tests

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Part of this code will go away again with #3229 but this makes sure the existing code actually works as expected.

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@danmar @amai2012

Since missingIncludeSystem was already potentially shadowed by the other warning I wonder if we should split them into two separate checks so you have more control over them. Usually you don't care about the missing system includes at all and Cppcheck works fine without them so you want to suppress the warning anyways.

And maybe missingInclude should be a warning instead of information since a missing local header has an effect on the analysis results and may even lead to false positives. So it is something we want the user to fix.

[firewave]

This should be okay to merge for now.