Enable test log collection from remote debug servers

Summary:
We already have the ability to collect the server logs when doing local debugging. This enables
the collection of remote logs as well. This relies on specifying a relative path "server.log" for
LLDB_DEBUGSERVER_LOG_FILE when starting remote platform. Since we always set the platform working
directory to a fresh folder to avoid conflicts, the actual file path will always be different and
we can pick the logs up from there.

Reviewers: tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D16322

llvm-svn: 258414