Move packet construction from GDBRemoteRegisterContext go the communication class

Summary:
When saving/restoring registers the GDBRemoteRegisterContext class was manually constructing
the register save/restore packets. This creates appropriate helper functions in
GDBRemoteCommunicationClient, and switches the class to use those. It also removes what a
duplicate packet send in some of those functions, a thing that I can only attribute to a bad
merge artefact.

I also add a test framework for testing gdb-remote client functionality and add tests for the new
functions I introduced. I'd like to be able to test the register context changes in isolation as
well, but currently there doesn't seem to be a way to reasonably construct a standalone register
context object, so we'll have to rely on the end-to-end tests to verify that.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D23553

llvm-svn: 278915