Fix tests under GETTEXT_POISON on pack-object

Fix tests under GETTEXT_POISON on pack-object

Use the i18n-specific test functions in test scripts for pack-object.
This issue was was introduced in v1.7.10.2-556-g46140:

    46140 index-pack: use streaming interface for collision test on large blobs
    cf2ba pack-objects: use streaming interface for reading large loose blobs

and been broken under GETTEXT_POISON=YesPlease since.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>