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.

rest-api-spec/src/main/resources/rest-api-spec/test/indices.get/10_basic.yml

@@ -162,6 +162,9 @@ setup:
 
 ---
 "Should return an exception when querying invalid indices":
+ - skip:
+     version: " - 5.99.99"
+     reason: "bad request logic added in 6.0.0"
 
  - do:
      catch: bad_request