Fix NO_NETWORK policy causing guests to lose progress; don't fetch example.com.

* caja.policy.net.NO_NETWORK.fetch now invokes its callback rather than
  ignoring it.
  Fixes <https://code.google.com/p/google-caja/issues/detail?id=1579>.
* test-cajajs-invocation testBuilderApiNetNone checks that <script src>
  and XHR fail sensibly under NO_NETWORK.
* In default test driver, use domain "caja-test-url.test" instead of
  "example.com" for rewriting, and don't fetch any such urls if
  they go to the fetcher (not all do). The TLD ".test" is reserved
  by RFC 2606.
  Fixes <https://code.google.com/p/google-caja/issues/detail?id=1680>.

@r5419

[ihab.awad, 2013-05-22 23:18:23]

lgtm

https://codereview.appspot.com/9638047/diff/1/tests/com/google/caja/plugin/es...
File tests/com/google/caja/plugin/es53-test-cajajs-invocation.js (right):

https://codereview.appspot.com/9638047/diff/1/tests/com/google/caja/plugin/es...
tests/com/google/caja/plugin/es53-test-cajajs-invocation.js:287: 'try {
xhr.open("http://localhost/"); } catch (e) { r(""+e); }' +
nit: space around "+" operator.