testrunner: refactored `givenACodeSampleToTokenize` into `SimpleTokenizer` and `SimpleTokenList` / several related cleanups by firewave · Pull Request #6084 · cppcheck-opensource/cppcheck

I didn't realize I already did this in #5299 - so adding some more changes from that.

This will also conflict with #6049.

This should be integrated into the fixture and also not use a wrapper so we can replace even more redundant code but I haven't figured that out yet.
This is done mainly so I can (hopefully) finally land #5299.