tests: fix fraction comparison in sort-float

tests: fix fraction comparison in sort-float

Problem reported by Cosima Neidahl <https://bugs.gnu.org/78985#13>.
* tests/sort/sort-float.sh: At top level, use C locale at first.
(dbl_minima_order): Assume C locale.
Use string comparison for the fractional parts.

2025-07-10  Paul Eggert  <eggert@cs.ucla.edu>

tests: fix integer overflow in sort-float
Problem reported by Cosima Neidahl <https://bugs.gnu.org/78985>.
* tests/sort/sort-float.sh (dbl_minima_order):
Use expr instead of test, to avoid problems with integers
too large for the shell.