[yugabyte#20514] docdb: Fix flaky test RaftConsensusITest.TestExpiredOperationWithSchemaChange and enable this test on fastdebug

Summary:
After we move from centos-gcc11-debug to alma8-gcc11-debug, RaftConsensusITest.TestExpiredOperationWithSchemaChange was constantly broken on alma8-gcc11-debug due to test bug.
In the test, the intention was to stuck the old leader ts, so that schema change operation can be successfully done on the new leader. However, cluster_->tablet_server(0) doesn't always give us the leader ts. After fixing this line of code, the test passed on all builds. This bug also led us to disable the test on fastdebug in the past. After the fix, we can re-enble this test on fastdebug.

Test Plan:
./yb_build.sh --cxx-test integration-tests_raft_consensus-itest --gtest_filter RaftConsensusITest.TestExpiredOperationWithSchemaChange --gcc11 -n 100
./yb_build.sh fastdebug --cxx-test integration-tests_raft_consensus-itest --gtest_filter RaftConsensusITest.TestExpiredOperationWithSchemaChange --gcc11 -n 100

Reviewers: rthallam, qhu

Reviewed By: qhu

Subscribers: ybase, bogdan

Differential Revision: https://phorge.dev.yugabyte.com/D31647

Author: yusong-yan

src/yb/integration-tests/raft_consensus-itest.cc

@@ -2010,7 +2010,7 @@ TEST_F(RaftConsensusITest, TestReplicaBehaviorViaRPC) {
   }
 }
 
-TEST_F(RaftConsensusITest, YB_DISABLE_TEST_IN_FASTDEBUG(TestExpiredOperationWithSchemaChange)) {
+TEST_F(RaftConsensusITest, TestExpiredOperationWithSchemaChange) {
   ANNOTATE_UNPROTECTED_WRITE(FLAGS_num_tablet_servers) = 3;
 
   vector<string> ts_flags = {
@@ -2030,8 +2030,8 @@ TEST_F(RaftConsensusITest, YB_DISABLE_TEST_IN_FASTDEBUG(TestExpiredOperationWith
   ASSERT_OK(WaitForServersToAgree(MonoDelta::FromSeconds(10), tablet_servers_, tablet_id_, 1));
 
   // Make the the prepare thread to delay.
-  auto* const ts = cluster_->tablet_server(0);
-  ASSERT_OK(cluster_->SetFlag(ts, "TEST_block_prepare_batch", "true"));
+  auto* const stuck_ts = cluster_->tablet_server_by_uuid(tservers[0]->uuid());
+  ASSERT_OK(cluster_->SetFlag(stuck_ts, "TEST_block_prepare_batch", "true"));
   // And the following write will also get delayed in prepare phase.
   ASSERT_NOK(WriteSimpleTestRow(
       tservers[0], tablet_id_, kTestRowKey, kTestRowIntVal, "foo", MonoDelta::FromSeconds(2)));
@@ -2068,7 +2068,7 @@ TEST_F(RaftConsensusITest, YB_DISABLE_TEST_IN_FASTDEBUG(TestExpiredOperationWith
 
   // The previous delayed write operation's term and schema version is less than the current.
   // Because of smaller term, we expect WriteQuery::Finish() will skip CHECK on schema version.
-  ASSERT_OK(cluster_->SetFlag(ts, "TEST_block_prepare_batch", "false"));
+  ASSERT_OK(cluster_->SetFlag(stuck_ts, "TEST_block_prepare_batch", "false"));
   SleepFor(MonoDelta::FromMilliseconds(1000));
 }