Refactor RecursiveASTVisitor test for post-order traversal

Summary:
The new test is now in the right directory with the other ASTVisitor tests and uses
now the provided TestVisitor framework.

Subscribers: hintonda, v.g.vassilev, klimek, cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D37557

llvm-svn: 323310