perf: export some important test-lib variables

perf: export some important test-lib variables

The only bug right now is that $GIT_TEST_CMP is needed for test_cmp to
work.

However, we also export the three most important paths for tests:

  TEST_DIRECTORY
  TRASH_DIRECTORY
  GIT_BUILD_DIR

Since they are available within test_expect_success, a future test
writer may expect them to also be defined in test_perf.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>