Add unit tests for cppcheckdata.py

[pfultz2]

This adds unit tests for cppcheckdata.py which runs with pytest. It will also run these tests with cmake automatically if python is available, so this should run on the CI.

The unit tests did surface a couple of problems, but I didnt fix it this PR to keep the scope smaller. Instead I just marked the test with pytest.mark.xfail for now. I can do a follow up PR to fix these issues:

1. Token.isBoolean is dead code. Tokenizer::dump (lib/tokenize.cpp:6179) checks tok->isName() before isBoolean(), and eBoolean tokens count as names (lib/token.h:397), so true/false are always dumped as type="name" and the type="boolean" branch at tokenize.cpp:6199 is unreachable for them(I see there is TODO: "true"/"false" aren't really a name...)
2. Line suppressions match every line. The final "other suppression" fallback in Suppression.isMatch() (cppcheckdata.py:1007) doesn't check that lineNumber is unset, so a suppression for line 5 also matches line 6. The fix would be a one-line self.lineNumber is None guard.
3. In C++ Token::Match, !!x also matches when there is no token at all (null), but Python match() fails when the token list ends: match(last_brace, '} !!x') returns false.