Edits to David's remoteString3 test

David's remoteString3 future recently started passing for gasnet-fast
testing, so Sung unfuturized it.  However, it still seems to fail for
gasnet-everything.  For that reason, I'm refuturizing it, but with a
skipif to avoid false positives in the gasnet-fast case.  Simultaneously,
I've added a clone of the test to lock in the gasnet-fast behavior since
it's working there for the time being.  I have a question out to Sung
wondering what would cause the difference.

When remoteString3.future is retired, its skipif should be removed,
as should the new remoteString3-seg-fast.chpl test.