Summary:
Adding tests that perform flush and compaction of the sys catalog during various phase of the ysql major catalog upgrade.

Fixes #24948
Jira: DB-14086

Test Plan:
YsqlMajorUpgradeRpcsTest, CompactSysCatalogAfterUpgrade
YsqlMajorUpgradeRpcsTest, CompactSysCatalogAfterRollback
YsqlMajorUpgradeRpcsTest, CompactSysCatalogAfterEveryPhase

Reviewers: tfoucher

Reviewed By: tfoucher

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D40036