Make sure index is healthy before calling _frozen in ReindexDatastreamIndexTransportActionIT (elastic#122623)

Author: masseyke

x-pack/plugin/migrate/src/internalClusterTest/java/org/elasticsearch/xpack/migrate/action/ReindexDatastreamIndexTransportActionIT.java

@@ -611,7 +611,7 @@ public void testTsdbStartEndSet() throws Exception {
     public void testIndexUnfrozen() {
         var sourceIndex = randomAlphaOfLength(20).toLowerCase(Locale.ROOT);
         safeGet(indicesAdmin().create(new CreateIndexRequest(sourceIndex)));
-
+        ensureHealth(sourceIndex);
         // add doc with timestamp
         String time = DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER.formatMillis(System.currentTimeMillis());
         var doc = String.format(Locale.ROOT, "{\"%s\":\"%s\"}", DEFAULT_TIMESTAMP_FIELD, time);