Count through the primary in list of strings test

A previous attempt to address a race condition in this test set wait for
active shards to all. However, there might not be any replicas if the
test is only running with one node so we end up waiting
forever. Instead, to address the intial race condition, we just count
through the primary.

rest-api-spec/src/main/resources/rest-api-spec/test/bulk/20_list_of_strings.yaml

@@ -1,10 +1,5 @@
 ---
 "List of strings":
-  - do:
-      indices.create:
-        index: text_index
-        wait_for_active_shards: all
-
   - do:
       bulk:
         refresh: true
@@ -16,6 +11,8 @@
 
   - do:
       count:
+        # we count through the primary in case there is a replica that has not yet fully recovered
+        preference: _primary
         index: test_index
 
   - match: {count: 2}