lldb-server tests: Add support for testing debugserver

Summary:
This adds support for running the lldb-server test suite (currently consisting
of only one test) against the debugserver. Currently, the choice which binary
to test is based on the host system. This will need to be replaced by something
more elaborate if/when lldb-server starts supporting debugging on darwin.

I need to make a couple of tweaks to the test client to work with debugserver:
- debugserver has different command-line arguments - launching code adjusted to
  handle that
- debugserver sends duplicate "medata" fields in the stop reply packet -
  adjusted stop-reply parsing code to handle that
- debugserver replies to the k packet instead of just dropping the connection -
  stopping code adjusted, although we should probably consider aligning the
  behavior of the two stubs in this case

Reviewers: jmajors, beanz

Subscribers: srhines, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D35311

llvm-svn: 316010