Modify dotest.py to be able to run without an lldb build.

Summary: This will ease llgs development a bit by not requiring an lldb/lldb.py build to launch the tests. Also, we can now use LLDB_DEBUGSERVER_PATH to point to a debug server to use to run the tests. I used that to point to a ds2 build and run llgs tests against ds2.

Reviewers: clayborg, tfiala, vharron

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D6554

llvm-svn: 225549