Summary:
Use namespace_name instead of yugabyte in ExecutePgFile.

Also adding a sanity check to ensure that the tests are running properly
Jira: DB-15127

Test Plan: XClusterPgRegressDDLReplicationTest

Reviewers: mlillibridge, xCluster, hsunder

Reviewed By: mlillibridge

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D41608