socket tests: Serialize sonewconn_overflow

It relies on being able inspect the dmesg buffer to see whether the
kernel logged certain messages. If it's interleaved with tests that
generate large amounts of console output (e.g., GELI tests), then it can
fail spuriously.

MFC after: 2 weeks

(cherry picked from commit ab27e232b14c07317642a2e4c546c9b71bd9c00b)