tests: use "$TEST_DIRECTORY" instead of ".."

tests: use "$TEST_DIRECTORY" instead of ".."

The $TEST_DIRECTORY variable allows tests to find the
top-level test directory regardless of the current working
directory.

In the past, this has been used to accomodate tests which
change directories, but it is also the first step to being
able to move trash directories outside of the
$TEST_DIRECTORY hierarchy.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>