tests: provide $TRASH_DIRECTORY variable

tests: provide $TRASH_DIRECTORY variable

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: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>