Add tests for sequences of callbacks that RecursiveASTVisitor produces

Summary:
These tests show a bug: post-order traversal introduces an extra call to
WalkUp*, that is not present in pre-order traversal. I'm fixing this bug
in a follow-up commit.

Reviewers: ymandel, eduucaldas

Reviewed By: ymandel, eduucaldas

Subscribers: gribozavr2, mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82485