Improve reliability of some tests

[smoelius]

This PR proposes the following changes to the tests:

• Add CALLED_BACK flags in several places to ensure callbacks are actually executed.
• Call unwrap on the result of the set_ssl_context call to ensure the call was successful.
• Rename connector_no_hostname_can_disable_verify to connector_can_disable_verify, and furthermore:
  • remove the call to verify_hostname (unnecessary since verification of the server's certificate is disabled);
  • change the connected-to hostname from foobar to fizzbuzz to emphasize that it is not checked.

[sfackler]

Thanks!