t9819: don't use test_must_fail with p4

t9819: don't use test_must_fail with p4

We were using `test_must_fail p4` to test that the p4 command failed 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 p4` with `! p4` and assume that the `p4` command does
not die unexpectedly.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>