No test ever ran the part of subdirJob that refuses to descend across
a filesystem boundary under -x. The new tests call subdirJob directly
with a made-up device for the operand, so no second real filesystem is
needed. They cover refusal across a boundary, skipping the check when
the operand's device is unknown, the warning when the subdirectory
cannot be statted, and crossing by default without -x. When descent is
accepted, the whole returned job is compared, so losing the operand's
device on the way down fails a test. scan.go is unchanged.
Model: opus-4-8 (implementation); opus-5-5 (rebase)
sneak