[clang-doc] Add unit tests for merging

Adds unit tests for the merging logic in Respresentation.cpp.

This is part of a move to convert clang-doc's tests to a more
maintainable unit test framework, with a smaller number of integration
tests to maintain and more granular failure feedback.

Differential Revision: https://reviews.llvm.org/D53083

llvm-svn: 344652