Shouldn't hit external sites without NETWORK_TESTING set

e.g. 
```
lives-ok { $res = $ua.get('http://httpbin.org/status/404') }, "no exception - expect 404";
```

from t/\*82\* - I just had a lengthy zef install fail because this test returned a 502 briefly. During a rerun with ``--serial`` install activated, it worked fine.

NETWORK_TESTING seems to be the standard in this kind of testing.


