Fix race condition in cat shards test

This commit fixes a test bug in the cat shards REST test. In
particular, there was a race condition in the test that would cause the
test to sometimes fail. The race condition is that some of the shards
would go to state STARTED after the sync flush was issued. These shards
would (correctly) show up in the output as having state started but
without a sync_id. However, the expected output was written to only
look for shards that have state STARTED and a sync_id, or shards that
are still INITIALIZING or are UNASSIGNED and (of course) do not have a
sync_id. The best approach here is to just simplify the test.

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

@@ -91,12 +91,11 @@
         body:
           settings:
             number_of_shards: 5
-            number_of_replicas: 1
+            number_of_replicas: 0
 
   - do:
       cluster.health:
-        wait_for_status: yellow
-        wait_for_relocating_shards: 0
+        wait_for_status: green
   - do:
       indices.flush_synced:
         index: sync_id_test
@@ -109,7 +108,7 @@
         h: index,state,sync_id
   - match:
       $body: |
-               /^(sync_id_test\s+(STARTED\s+[A-Za-z0-9_\-]{20}|(INITIALIZING|UNASSIGNED)\s+)\s+\n){10}$/
+               /^(sync_id_test\s+STARTED\s+[A-Za-z0-9_\-]{20}\s+\n){5}$/
 
   - do:
       indices.delete: