[ASTImporter][NFC] Improve test coverage

All three cases were imported correctly.
For BlockDecls, correctly means that we don't support importing them, thus an
error is the expected behaviour.

 - BlockDecls were not yet covered. I know that they are not imported but the
   test at least documents it.
 - Default values for ParmVarDecls were also uncovered.
 - Importing bitfield FieldDecls were imported correctly.

Reviewed By: martong, shafik

Differential Revision: https://reviews.llvm.org/D99576