test-lib: Use $TEST_DIRECTORY or $GIT_BUILD_DIR instead of $(pwd) and ../

test-lib: Use $TEST_DIRECTORY or $GIT_BUILD_DIR instead of $(pwd) and ../

Change the redundant calls to $(pwd) to use $TEST_DIRECTORY
instead. None of these were being executed after we cd'd somewhere
else so they weren't actually needed.

This also makes it easier to add support for overriding the test
library location and run tests in a different directory than t/.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>