t3902-*.sh: Skip all tests rather than each test

Each test in this file is skipped if the TABS_IN_FILENAMES test
prerequisite is set. Use the 'skip_all' facility at the head of
the file to skip all of the tests instead.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>