[#29197] YSQL: fix TestConflictAbortViaOldHeartbeatBeforeNewHeartbeat

Summary:
Read inside serializable transaction in the `WarmupTablespaceCache` helper function introduces extra delay in `asan`/`tsan` in several unit test (these tests add additional delay in distributed txn for testing purposes via GFlags). As a result units test are executed for a long time and also may fail due to timing issue.

The fix is to make a read from default transaction (read committed or repeatable read) which doesn't use distributed transaction for read operations.
Jira: DB-18960

Test Plan: Jenkins

Reviewers: esheng, bkolagani, #db-approvers

Reviewed By: bkolagani, #db-approvers

Subscribers: svc_phabricator, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D48049

Author: d-uspenskiy

src/yb/yql/pgwrapper/geo_transactions_test_base.cc

@@ -388,11 +388,10 @@ Result<std::vector<TabletId>> GeoTransactionsTestBase::GetStatusTablets(
 
 Status GeoTransactionsTestBase::WarmupTablespaceCache(
     pgwrapper::PGConn& conn, std::string_view table) {
-  // Force tablespace information into cache. Since SERIALIZABLE replicates reads, this also
-  // serves to ensure transaction is not reused (for object locking enabled cases).
-  RETURN_NOT_OK(conn.StartTransaction(IsolationLevel::SERIALIZABLE_ISOLATION));
-  RETURN_NOT_OK(conn.FetchFormat("SELECT * FROM $0 LIMIT 1", table));
-  return conn.RollbackTransaction();
+  // Force tablespace information into cache.
+  // The purpose of ROLLBACK is to to ensure that in case object locking is enabled YB txn
+  // will not be reused.
+  return conn.ExecuteFormat("BEGIN;SELECT * FROM $0 LIMIT 1;ROLLBACK", table);
 }
 
 } // namespace yb::client