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, 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) ||                                   \
        F_ISSET(session, WT_SESSION_INTERNAL),                                            \
"schema lock acquired during role transition");

With asynchronous step-up, concurrent user-driven schema operations should now be permitted. Remove the step-up assertion and update the associated python test test_layered_stepup12.py to cover step-up only (the step-down counterpart is tracked in WT-17880).