Skip bad request REST test on pre-6.0
This commit adds a skip for the bad request REST test on pre-6.0
nodes. Previously, a request for /_(.*) where $1 is not an existing
endpoint would return a 404. This is because the request would be
treated as a get index request for an index named _$1. However, an index
can never start with "_" so logic was added to detect this and return a
400 instead as this should be treated as a bad request. During the
mixed-cluster BWC tests, a node running pre-6.0 code will still return a
404 though. Therefore, this test needs to skipped in such a
mixed-cluster scenario.
Jason Tedor