[TEST] Wait for yellow since some shards might not be started

In this test we only index a handful of docs so if we have more shards
than docs we might fail on the `assertSearchResult` since not all shards
are started but results are just fine.

src/test/java/org/elasticsearch/search/query/ExistsMissingTests.java

@@ -103,7 +103,7 @@ public class ExistsMissingTests extends ElasticsearchIntegrationTest {
         expected.put("foobar", 0);
 
         final long numDocs = client().prepareSearch("idx").execute().actionGet().getHits().totalHits();
-
+        ensureYellow("idx");
         for (Map.Entry<String, Integer> entry : expected.entrySet()) {
             final String fieldName = entry.getKey();
             final int count = entry.getValue();