t9902: disable test on the list of merge-strategies under GETTEXT_POISON

t9902: disable test on the list of merge-strategies under GETTEXT_POISON

The code to learn the list of merge strategies from the output of
"git merge -s help" forces C locale, so that it can notice the
message shown to indicate where the list starts in the output.

However, GETTEXT_POISON build corrupts its output even when run in
the C locale, and we cannot expect this test to succeed.

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