Most scripts don't care about the absolute path to the trash
directory. The one exception was t4014 script, which pieced
together $TEST_DIRECTORY and $test itself to get an absolute
directory.

Instead, let's provide a $TRASH_DIRECTORY which specifies
the same thing. This keeps the $test variable internal to
test-lib.sh and paves the way for trash directories in other
locations.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>