Summary:
Add a new test that:
1. In PG11, creates a schema and 3 tables. One in the default schema, one explicitly in public, and one in the new schema.
2. Upgrade the cluster to mixed mode and check that the three tables are present on both PG15 nodes and PG11 nodes.
3. Finalize the upgrade and recheck the three tables. Create three similar tables and validate that all 6 tables are present.
Checking the tables are present involves:
* checking the relname and namespace name are as expected in pg_class
* checking that a trivial select from the table succeeds
Test Plan:
./yb_build.sh --cxx-test pg15_upgrade-test --gtest_filter Pg15UpgradeTest.Schemas -n 50
Jenkins: test regex: .*Pg15UpgradeTest.*
Reviewers: tfoucher, hsunder, fizaa
Reviewed By: tfoucher
Subscribers: yql
Differential Revision: https://phorge.dev.yugabyte.com/D38594