Increase timeout for index migration in FullClusterRestartSystemIndexCompatibilityIT (elastic#127710)

This test occasionally fails on version 8.19 clusters when we are waiting
for system index migration to finish. This changes the wait time from the
10s default to 30s to account for the occasional super slow cluster in tests.

Closes elastic#127245

Author: cbuescher

qa/lucene-index-compatibility/src/javaRestTest/java/org/elasticsearch/lucene/FullClusterRestartSystemIndexCompatibilityIT.java

@@ -22,6 +22,7 @@
 import java.io.IOException;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.TimeUnit;
 
 import static org.hamcrest.Matchers.equalTo;
 
@@ -95,7 +96,7 @@ public void testAsyncSearchIndexMigration() throws Exception {
                 } catch (IOException e) {
                     throw new AssertionError("System feature migration failed", e);
                 }
-            });
+            }, 30, TimeUnit.SECONDS);
 
             // check search results from n-2 search are still readable
             assertAsyncSearchHitCount(async_search_ids.get("n-2_id"), numDocs);