tests: Skip errors-server-oversize with nbdkit 1.32.4

Stable nbdkit release 1.32.4 backported the addition of exposing
max_known_status= in sh/eval --dump-plugin output (see nbdkit commits
62868cae and fd1c90e6), but not support for exit status 6 (b1ba5275).
Enhance our probe for a new-enough nbdkit to properly skip this setup.

Fixes: 122322cd ("tests: Test server response to oversize requests", v1.15.8)