Added tests for RecursiveASTVisitor for AST nodes that are special cased

Summary:
RecursiveASTVisitor has special code for handling operator AST nodes,
specifically, unary, binary, and compound assignment operators. In this
change I'm adding tests for operator AST nodes that follow the existing
pattern of tests for the CallExpr node (an AST node that triggers the
common code path).

Reviewers: ymandel, eduucaldas

Reviewed By: ymandel, eduucaldas

Subscribers: gribozavr2, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82875