t9129: fix UTF-8 locale detection

t9129: fix UTF-8 locale detection

The UTF-8 prerequisite test checked explicitly for en_US.utf8 in the
output from "locale -a", but the tests that are actually protected by the
prerequisite were asking LC_ALL=en_US.UTF-8 from the system.

This inconsistency leads the tests to fail on platforms that do not know
both en_US.UTF-8 and en_US.utf8 (thanks you, Yann Droneaud, for bringing
this up with an initial patch).

Instead, pick a locale with ".UTF-8" (with or without hyphen, spelled in
either upper or lowercase) in its name from "locale -a" output, and use it
for running the test.

Signed-off-by: Junio C Hamano <gitster@pobox.com>