tests: Fix build with clang

tests: Fix build with clang

clang doesn't like mode_t type as an argument to va_arg():

error: second argument to 'va_arg' is of promotable type 'mode_t' (aka
'unsigned short'); this va_arg has undefined behavior because arguments
will be promoted to 'int'

    mode = va_arg(ap, mode_t);
                      ^~~~~~

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>