Added capability to avoid running tests in clusters without the fix

Author: ivancea

modules/aggregations/src/yamlRestTest/resources/rest-api-spec/test/aggregations/sig_terms_nested.yml

@@ -1,6 +1,11 @@
 setup:
   - requires:
-      test_runner_features: close_to
+      capabilities:
+        - method: POST
+          path: /_search
+          capabilities: [ significant_terms_on_nested_fields ]
+      test_runner_features: [capabilities, close_to]
+      reason: "bug fix"
 
   - do:
       indices.create:

server/src/main/java/org/elasticsearch/rest/action/search/SearchCapabilities.java

@@ -50,6 +50,8 @@ private SearchCapabilities() {}
 
     private static final String SIGNIFICANT_TERMS_BACKGROUND_FILTER_AS_SUB = "significant_terms_background_filter_as_sub";
 
+    private static final String SIGNIFICANT_TERMS_ON_NESTED_FIELDS = "significant_terms_on_nested_fields";
+
     public static final Set<String> CAPABILITIES;
     static {
         HashSet<String> capabilities = new HashSet<>();
@@ -69,6 +71,7 @@ private SearchCapabilities() {}
         capabilities.add(HIGHLIGHT_MAX_ANALYZED_OFFSET_DEFAULT);
         capabilities.add(INDEX_SELECTOR_SYNTAX);
         capabilities.add(SIGNIFICANT_TERMS_BACKGROUND_FILTER_AS_SUB);
+        capabilities.add(SIGNIFICANT_TERMS_ON_NESTED_FIELDS);
         CAPABILITIES = Set.copyOf(capabilities);
     }
 }