tests: Fix mode_t usage with va_arg()

tests: Fix mode_t usage with va_arg()

Clang complains about it:

  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'
  [-Werror,-Wvarargs]

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

Work around the issue by passing int to va_arg() and casting
its return value to mode_t afterwards.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>