[TEST] added wait for relocationg shards to cat.shards REST test

Adding the second index might cause relocation of existing shards depending on the number of nodes available, let's wait for relocation to be finished before going ahead and checking the cat shards outpu

rest-api-spec/test/cat.shards/10_basic.yaml

@@ -35,6 +35,7 @@
   - do:
        cluster.health:
          wait_for_status: yellow
+         wait_for_relocating_shards: 0
 
   - do:
       cat.shards: {}