[lldb] Generic base for testing gdb-remote behavior

Summary:
Adds new utilities that make it easier to write test cases for lldb acting as a client over a gdb-remote connection.

- A GDBRemoteTestBase class that starts a mock GDB server and provides an easy way to check client packets
- A MockGDBServer that, via MockGDBServerResponder, can be made to issue server responses that test client behavior.
- Utility functions for handling common data encoding/decoding
- Utility functions for creating dummy targets from YAML files

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Split from the review at https://reviews.llvm.org/D42145, which was a new feature that necessitated the new testing capabilities.

Reviewers: clayborg, labath

Reviewed By: clayborg, labath

Subscribers: hintonda, davide, jingham, krytarowski, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D42195
Patch by Owen Shaw <llvm@owenpshaw.net>

llvm-svn: 323636