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: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

tests/nssmock.c

@@ -87,7 +87,7 @@ open(const char *path, int flags, ...)
         va_list ap;
         mode_t mode;
         va_start(ap, flags);
-        mode = va_arg(ap, int);
+        mode = (mode_t) va_arg(ap, int);
         va_end(ap);
         ret = real_open(newpath ? newpath : path, flags, mode);
     } else {

tests/vircgroupmock.c

@@ -731,7 +731,7 @@ int open(const char *path, int flags, ...)
         va_list ap;
         mode_t mode;
         va_start(ap, flags);
-        mode = va_arg(ap, mode_t);
+        mode = (mode_t) va_arg(ap, int);
         va_end(ap);
         ret = real_open(newpath ? newpath : path, flags, mode);
     } else {

tests/virpcimock.c

@@ -983,7 +983,7 @@ open(const char *path, int flags, ...)
         va_list ap;
         mode_t mode;
         va_start(ap, flags);
-        mode = va_arg(ap, mode_t);
+        mode = (mode_t) va_arg(ap, int);
         va_end(ap);
         ret = real_open(newpath ? newpath : path, flags, mode);
     } else {

tests/virtestmock.c

@@ -180,7 +180,7 @@ int open(const char *path, int flags, ...)
         va_list ap;
         mode_t mode;
         va_start(ap, flags);
-        mode = va_arg(ap, mode_t);
+        mode = (mode_t) va_arg(ap, int);
         va_end(ap);
         ret = real_open(path, flags, mode);
     } else {