Fix fix reference cycle in test col_accessor_map callback

Fix fix reference cycle in test col_accessor_map callback

This was making the tester's DESTROY method try to reconnect to delete
tables during global destruction, causing crashes (including
segfaults).

There's still something else causing causing occasional segfaults in
XS_DBI_dispatch (called for DBI::common::DESTROY) during global
destruction.

In passing, use curry everywhere appropriate.