[ILM][TEST] increase assertBusy timeout (elastic#36864)

the testFullPolicy and testMoveToRolloverStep tests
are very important tests, but they sometimes timeout
beyond the default 10sec wait for shrink to occur.
This commit increases one of the assertBusys to
20 seconds

x-pack/plugin/ilm/qa/multi-node/src/test/java/org/elasticsearch/xpack/indexlifecycle/TimeSeriesLifecycleActionsIT.java

@@ -46,6 +46,7 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.concurrent.TimeUnit;
 import java.util.function.Supplier;
 
 import static java.util.Collections.singletonMap;
@@ -99,7 +100,7 @@ public void testFullPolicy() throws Exception {
         // asserts that rollover was called
         assertBusy(() -> assertTrue(indexExists(secondIndex)));
         // asserts that shrink deleted the original index
-        assertBusy(() -> assertFalse(indexExists(originalIndex)));
+        assertBusy(() -> assertFalse(indexExists(originalIndex)), 20, TimeUnit.SECONDS);
         // asserts that the delete phase completed for the managed shrunken index
         assertBusy(() -> assertFalse(indexExists(shrunkenOriginalIndex)));
     }
@@ -177,7 +178,7 @@ public void testMoveToRolloverStep() throws Exception {
         // asserts that rollover was called
         assertBusy(() -> assertTrue(indexExists(secondIndex)));
         // asserts that shrink deleted the original index
-        assertBusy(() -> assertFalse(indexExists(originalIndex)));
+        assertBusy(() -> assertFalse(indexExists(originalIndex)), 20, TimeUnit.SECONDS);
         // asserts that the delete phase completed for the managed shrunken index
         assertBusy(() -> assertFalse(indexExists(shrunkenOriginalIndex)));
     }