t9123: use test_when_finished for cleanup

t9123: use test_when_finished for cleanup

Move the setup logic into a 'test_expect_success' block.
This ensures that the code is properly tracked by the test harness.

Additionally, we use the 'test_when_finished' helper at the start of
the block to ensure that the 'import' directory is removed even if the
test fails.
This is cleaner than the previous manual 'rm -rf import' approach.

Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>