In WT-17656 we added checks to the WT_WITH_SCHEMA_LOCK(_NOWAIT) macros that assert that we are not currently in the middle of a role-transition. This was not allowed to happen with synchronous step-ups/step-downs, as a concurrent user-initiated schema operation could potentially interfere with the process (e.g. BF-42866).

        WT_ASSERT_ALWAYS(session,                                                             \
          !F_ISSET_ATOMIC_32(                                                                 \
            S2C(session), WT_CONN_RECONFIGURING_STEP_UP | WT_CONN_RECONFIGURING_STEP_DOWN) || \
            F_ISSET(session, WT_SESSION_INTERNAL),                                            \
          "schema lock acquired during role transition"); 

With asynchronous step-up/step-down, concurrent user-driven schema operations should now be permitted, and we should remove the assertions along with the associated python test test_layered_stepup12.py.