test-json: do not pass ephemeral array as intializer to JSON_BUILD_STRV

Fixes #11600.

The code was effectively doing:
  json_build(..., ({ char **_x = ((char**) ((const char*[]) {"one", "two", "three", "four", NULL })); _x; }));
but there was no guarantee that the storage for the array that _x points to
survives pass the end of the block. Essentially, STRV_MAKE cannot be used
inline inside of a block like this.