[Transform] fix time rounding in TransformContinuousIT (#63113)

fix a time rounding problem in the test, due to rounding down to epoch
seconds instead of epoch millis

fixes #62951

x-pack/plugin/transform/qa/single-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/transform/integration/continuous/TransformContinuousIT.java

@@ -237,7 +237,10 @@ public class TransformContinuousIT extends ESRestTestCase {
 
             // start all transforms, wait until the processed all data and stop them
             startTransforms();
-            waitUntilTransformsReachedUpperBound(runDate.getEpochSecond() * 1000 + 1);
+
+            // at random we added between 0 and 999_999ns == (1ms - 1ns) to every data point, so we add 1ms, so every data point is before
+            // the checkpoint
+            waitUntilTransformsReachedUpperBound(runDate.toEpochMilli() + 1, run);
             stopTransforms();
 
             // TODO: the transform dest index requires a refresh, see gh#51154
@@ -387,11 +390,12 @@ public class TransformContinuousIT extends ESRestTestCase {
         }
     }
 
-    private void waitUntilTransformsReachedUpperBound(long timeStampUpperBoundMillis) throws Exception {
+    private void waitUntilTransformsReachedUpperBound(long timeStampUpperBoundMillis, int iteration) throws Exception {
         logger.info(
-            "wait until transform reaches timestamp_millis: {}",
+            "wait until transform reaches timestamp_millis: {} iteration: {}",
             ContinuousTestCase.STRICT_DATE_OPTIONAL_TIME_PRINTER_NANOS.withZone(ZoneId.of("UTC"))
-                .format(Instant.ofEpochMilli(timeStampUpperBoundMillis))
+                .format(Instant.ofEpochMilli(timeStampUpperBoundMillis)),
+            iteration
         );
         for (ContinuousTestCase testCase : transformTestCases) {
             assertBusy(() -> {