migration: Fix flaky TestIndexStatus
Created by: efritz
Fixes #33837 (closed). This PR disables a the flaky test in affected db-backcompat tests, and fixes the test's flakiness by polling an index status for up to one second instead of checking for an exclusive lock in postgres system tables. Apparently there is a race between a postgres backend acquiring that lock and checking for index creation status details.
Test plan
N/A.