Similarly to the LibCpp parser regression tests, these tests run the
preprocessor on the .cpp test files under
Userland/LibCpp/Tests/preprocessor, and compare the output with existing
.txt ground truth files.
Previously, we had a special ASTNode for class members,
"MemberDeclaration", which only represented fields.
This commit removes MemberDeclaration and instead uses regular
Declaration nodes for representing the members of a class.
This means that we can now also parse methods, inner-classes, and other
declarations that appear inside of a class.
For each .cpp file in the test suite data, there is a .ast file that
represents the "known good" baseline of the parser result.
Each .cpp file goes through the parser, and the result of
invoking `ASTNode::dump()` on the root node is compared to the
baseline to find regressions.
We also check that there were no parser errors when parsing the .cpp
files.
