Fix testKeepTranslogAfterGlobalCheckpoint

Read the last synced global checkpoint after flushing as
we might advance it during committing.

CI: https://gradle-enterprise.elastic.co/s/7o6qengg4gva2

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

@@ -4891,8 +4891,8 @@ protected void commitIndexWriter(IndexWriter writer, Translog translog, String s
                     engine.syncTranslog();
                 }
                 if (frequently()) {
-                    final long lastSyncedGlobalCheckpoint = Translog.readGlobalCheckpoint(translogPath, translogUUID);
                     engine.flush(randomBoolean(), true);
+                    final long lastSyncedGlobalCheckpoint = Translog.readGlobalCheckpoint(translogPath, translogUUID);
                     final List<IndexCommit> commits = DirectoryReader.listCommits(store.directory());
                     // Keep only one safe commit as the oldest commit.
                     final IndexCommit safeCommit = commits.get(0);