debug: residual_prototype_relocation test intermittently segfaults at a 2 MiB block base in the full gc test sequence #12137

In a debug build, gc::tests::residual_prototype_relocation::test_residual_prototype_owners_of_every_movable_kind_survive_a_copying_minor sometimes segfaults at a 2 MiB block base when run in the full gc test sequence: main 563c293 in 1 of 2 runs, and with the #12098 fix in 2 of 3. It passes in isolation 3 of 3. The faulting address is a 2 MiB block base, so it may interact with the region manager (#12132). Found during #12098.