t/test-lib.sh: fix TRASH_DIRECTORY handling

t/test-lib.sh: fix TRASH_DIRECTORY handling

After the location of $TRASH_DIRECTORY is adjusted by
$TEST_OUTPUT_DIRECTORY, we go on to use the $test variable to make the
trash directory and cd into it.  This means that when
$TEST_OUTPUT_DIRECTORY is not "." and an absolute --root has not been
specified, we do not remove the trash directory once the tests are
complete (remove_trash is set to $TRASH_DIRECTORY).

Fix this by always referring to the trash directory as $TRASH_DIRECTORY.

Signed-off-by: John Keeping <redacted>
Acked-by: Jeff King <redacted>
Acked-by: Thomas Rast <redacted>
Signed-off-by: Junio C Hamano <redacted>