Summary:
With f2f0d6f49f84c0916bc2834ad7d577da9e3c212e /D41402, EXPLAIN output displays table aliases. Concurrently, f40f9663f571215e3af87cd9cb04c9aee7f773be / D42809 added a new test but without incorporating the change in EXPLAIN output (because D41402 hadn't landed by then).

This broke the TestPgRegressPushdown test. Add the table alias to the expected output and get the test to pass.
Jira: DB-15993

Test Plan:
Jenkins: test regex: TestPgRegressPushdown*
   ./yb_build.sh --java-test 'org.yb.pgsql.TestPgRegressPushdown'

Close: #26616

Reviewers: stiwary, telgersma

Reviewed By: stiwary

Subscribers: svc_phabricator, yql

Differential Revision: https://phorge.dev.yugabyte.com/D42941