Implemented gdb-remote protocol tests for vCont;s and vCont;s:{thread}

Also added tests for presence of vCont;c, vCont;C, vCont;s, vCont;S as
returned by vCont? query.

Broke out single step functionality from TestLldbGdbServer into base class.
Used by new TestGdbRemoteSingleStep (using $s) and TestGdbRemote_vCont.

Also part of llgs wrap-up, see:
https://github.com/tfiala/lldb/issues/12