xargs: Convert tests to ATF

[des]

{ARG_MAX} is variable, don't hardcode it

[des]

consistent naming

[kevans]

usr.bin/xargs/tests/xargs_test.sh

3

I think you'd mentioned you got some permission here from @jmallett out-of-band, please mention that explicitly in the commit message.

[jmallett]

I'm completely out of the loop on what's technically useful here, but happy to confirm that @des did get my permission to add a copyright. I'm glad to see xargs getting some good attention!