[Transform] fix itermittent problem in TransformIndexerTests/TransformIndexerFailureHandlingTests (#70326)

fix a race condition in the test: the indexer thread might still be in the
process of shutting down, when the test thread triggers it again.

relates #69551
fixes #70297

Author: Hendrik Muhs

x-pack/plugin/transform/src/test/java/org/elasticsearch/xpack/transform/transforms/TransformIndexerFailureHandlingTests.java

@@ -341,10 +341,11 @@ public void testPageSizeAdapt() throws Exception {
 
         // run indexer a 2nd time
         final CountDownLatch secondRunLatch = indexer.newLatch(1);
-        indexer.start();
         assertEquals(pageSizeAfterFirstReduction, indexer.getPageSize());
         assertThat(indexer.getState(), equalTo(IndexerState.STARTED));
-        assertTrue(indexer.maybeTriggerAsyncJob(System.currentTimeMillis()));
+
+        // when the indexer thread shuts down, it ignores the trigger, we might have to call it again
+        assertBusy(() -> assertTrue(indexer.maybeTriggerAsyncJob(System.currentTimeMillis())));
         assertThat(indexer.getState(), equalTo(IndexerState.INDEXING));
 
         secondRunLatch.countDown();