The beginnings of a gtest-based test framework.

Makes use of LLVM gtest support.  This does *not* run as part of
the lldb test suite.

I'm using it to start testing some components that
I'll be adding to the inner guts of NativeThreadLinux to more
maintainably handle thread states and deferred thread state notification.

Runs with default Makefile target "test" using gmake within a given
test directory (currently only test/c++/native_process/thread_state_coordinator).

The Makefile.rules currently assume it is using the LLVM gtest.  It works on
a canonical MacOSX dir structture (i.e. lldb, lldb/llvm, lldb/llvm/tools/clang).
It also works on Ubuntu assuming the standard dir layout of llvm, llvm/tools/clang,
llvm/tools/lldb.  In this case, it expects a directory called build-debug parallel
to the llvm source dir.  All directory assumptions can be overridden with
environment variables.  See test/c++/make/Makefile.rules for details.

We'll want to make this smarter in the future, particularly around finding the LLVM build
output dir.

llvm-svn: 218422