t9400: don't use test_must_fail with cvs

We are using `test_must_fail cvs` to test that the cvs command fails as expected. However, test_must_fail() is used to ensure that commands fail in an expected way, not due to something like a segv. Since we are not in the business of verifying the sanity of the external world, replace `test_must_fail cvs` with `! cvs` and assume that the cvs command does not die unexpectedly. Signed-off-by: Denton Liu <liu.denton@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>