test-lib: Allow overriding of TEST_DIRECTORY
test-lib: Allow overriding of TEST_DIRECTORY

Tests that test the test-lib.sh itself need to be executed in the
dynamically created trash directory, so we can't assume
$TEST_DIRECTORY is ../ for those.

As a side benefit this change also makes it easy for us to move the
t/*.sh tests into subdirectories if we ever want to do that.

Signed-off-by: Ævar Arnfjörð Bjarmason <0135fcc047f75af264c800977a73e353c6ba842d@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git