Fix InternalEngineTests.testSeqNoAndCheckpoints (#51630)

* Fix InternalEngineTests.testSeqNoAndCheckpoints

If we force flush while possibly triggering a merge the local checkpoint may change
from the expectation from the loop that just increments on every operation.

Closes #51604

server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -2102,7 +2102,7 @@ public class InternalEngineTests extends EngineTestCase {
                     id = randomFrom(indexedIds);
                     final Engine.Delete delete = new Engine.Delete(
                         id, newUid(id), UNASSIGNED_SEQ_NO, primaryTerm.get(),
-                        rarely() ? 100 : Versions.MATCH_ANY, VersionType.INTERNAL, PRIMARY, 0, UNASSIGNED_SEQ_NO, 0);
+                        rarely() ? 100 : Versions.MATCH_ANY, VersionType.INTERNAL, PRIMARY, System.nanoTime(), UNASSIGNED_SEQ_NO, 0);
                     final Engine.DeleteResult result = initialEngine.delete(delete);
                     if (result.getResultType() == Engine.Result.Type.SUCCESS) {
                         assertThat(result.getSeqNo(), equalTo(primarySeqNo + 1));
@@ -2120,7 +2120,7 @@ public class InternalEngineTests extends EngineTestCase {
                     final Engine.Index index = new Engine.Index(newUid(doc), doc,
                         UNASSIGNED_SEQ_NO, primaryTerm.get(),
                         rarely() ? 100 : Versions.MATCH_ANY, VersionType.INTERNAL,
-                        PRIMARY, 0, -1, false, UNASSIGNED_SEQ_NO, 0);
+                        PRIMARY, System.nanoTime(), -1, false, UNASSIGNED_SEQ_NO, 0);
                     final Engine.IndexResult result = initialEngine.index(index);
                     if (result.getResultType() == Engine.Result.Type.SUCCESS) {
                         assertThat(result.getSeqNo(), equalTo(primarySeqNo + 1));