Get rid of minus zero in box regression test.

Get rid of minus zero in box regression test.

Commit acdf2a8b added a test case involving minus zero as a box endpoint.
This is not very portable, as evidenced by the several older buildfarm
members that are failing on the test because they print minus zero as
just "0".  If there were any significant reason to test this behavior,
we could consider carrying a separate expected-file; but it doesn't look
to me like there's adequate justification to accept such a maintenance
burden.  Just change the test to use plain zero, instead.