t0000: set TEST_OUTPUT_DIRECTORY for sub-tests

t0000: set TEST_OUTPUT_DIRECTORY for sub-tests

Running t0000 produces more trash directories than expected and does
not clean up after itself:

    $ ./t0000-basic.sh
    [...]
    $ ls -d trash\ directory.*
    trash directory.failing-cleanup
    trash directory.mixed-results1
    trash directory.mixed-results2
    trash directory.partial-pass
    trash directory.test-verbose
    trash directory.test-verbose-only-2

These scratch areas for sub-tests should be under the t0000 trash
directory, but because TEST_OUTPUT_DIRECTORY defaults to
TEST_DIRECTORY, which is exported to help sub-tests find
test-lib.sh, the sub-test trash directories are created under the
toplevel t/ directory instead.  Because some of the sub-tests
simulate failures, their trash directories are kept around.

Fix it by explicitly setting TEST_OUTPUT_DIRECTORY appropriately for
sub-tests.

An alternative fix would be to pass the --root parameter that only
specifies where to put the trash directories, which would also work.
However, using TEST_OUTPUT_DIRECTORY is more futureproof in case
tests want to write more output in addition to the test-results/
(which are already suppressed in sub-tests using the HARNESS_ACTIVE
setting) and trash directories.

This fixes a regression introduced by 38b074d (t/test-lib.sh: fix
TRASH_DIRECTORY handling, 2013-04-14).  Before that commit, the
TEST_OUTPUT_DIRECTORY setting was not respected consistently so most
tests did their work in a "trash" subdirectory of the current
directory instead of the output dir.

Signed-off-by: Jeff King <peff@peff.net>
Clarified-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>