Export more test-related variables when running external tests

Export more test-related variables when running external tests

Add exporting TEST_DIRECTORY and TRASH_DIRECTORY to test_external, for
external tests to be able to find test script (and git sources), and
to find trash directory (usually with test repository in it).

Add also exporting GIT_TEST_LONG, so that external test can skip
time-intensive tests unless test is invoked with `--long' option.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>