tests: Avoid dependency on strtoumax.

tests: Avoid dependency on strtoumax.

Pretty easy to implement string-to-integer conversion in the tests
now that we have some helpers to do so that can be shared with the
main library code.

There is very little advantage to this change in the test programs,
as the reduced-range fallback is pretty much fine here.  However,
as there are no other users, getting rid of this also allows us to
remove the configure probes for strtoumax and its alternatives.