test-lib: don't set GIT_EXIT_OK before calling test_atexit_handler

test-lib: don't set GIT_EXIT_OK before calling test_atexit_handler

Change the control flow in test_done so that we'll set GIT_EXIT_OK=t
after we call test_atexit_handler(). This seems to have been a mistake
in 900721e15c4 (test-lib: introduce 'test_atexit', 2019-03-13). It
doesn't make sense to allow our "atexit" handling to call "exit"
without us emitting the errors we'll emit without GIT_EXIT_OK=t being
set.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>