release-combined.tested: readd boot.biosUsb test

It has been commented-out for some time and now removed
in commit 2de3caf0, but it seems reliable in past months:
https://hydra.nixos.org/job/nixos/trunk-combined/nixos.tests.boot.biosUsb.x86_64-linux/all
so I can't see why not re-add it.  Close #15690.