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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

t/t9400-git-cvsserver-server.sh

@@ -603,7 +603,7 @@ test_expect_success 'cvs server does not run with vanilla git-shell' '
 		cd cvswork &&
 		CVS_SERVER=$WORKDIR/remote-cvs &&
 		export CVS_SERVER &&
-		test_must_fail cvs log merge
+		! cvs log merge
 	)
 '