The -x descent rules in subdirJob had no test executing the actual
rejection: the only case proved -x skips nothing when everything is on
one filesystem. Add tests that call subdirJob directly with a fabricated
operand device, so no second real filesystem is needed:
- reject descent when the subdirectory is on a different device;
- bypass the check when the operand's own device is unknown;
- warn and refuse descent when the entry's Info() fails;
- cross the boundary by default (no -x).
Tests only; scan.go is unchanged.
Model: opus-4-8
sneak