scylla-2026.3.1: test/cluster: accept the ban notification when a join is rolled back
A node whose bootstrap or replace fails is rolled back by the topology coordinator through left_token_ring, and finish_left_token_ring_transition removes it from group0 and bans it. The joining node is racing that: it either observes its own failed request and aborts startup with the message the test expects, or receives the ban notification first and terminates on that instead. Both are legitimat…