[clang-format] Add a test for associative map proto buffer fields

Summary:
The test suite was missing a test about associative maps:
https://developers.google.com/protocol-buffers/docs/proto3#maps

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D34623

llvm-svn: 306386