golang: Skip test using syscall.Select on BSDs

./libnbd_590_aio_copy_test.go:169:13: assignment mismatch: 2 variables but syscall.Select returns 1 value

This is caused by the signature being different on BSD.  Might as well
skip this test on anything which isn't Linux.  More here:

https://github.com/golang/go/issues/34458