Mark IHBT#testFromAndToXContent as awaits fix

This commit marks InnerHitsBuilderTests#testFromAndToXContent as
awaiting a fix.

core/src/test/java/org/elasticsearch/index/query/InnerHitBuilderTests.java

@@ -87,6 +87,7 @@ public class InnerHitBuilderTests extends ESTestCase {
         }
     }
 
+    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/18166")
     public void testFromAndToXContent() throws Exception {
         for (int runs = 0; runs < NUMBER_OF_TESTBUILDERS; runs++) {
             InnerHitBuilder innerHit = randomInnerHits(true, false);