Add tests for holding back alerts

[howardjohn]

This is a PR against a PR (#147).

• Adds tests for both cases: deciding to writing the alert, and deciding to not write the alert. Note: the reason one would decide to not write the alert would typically be some check if its HTTP (if so, send HTTP error) else send the original alert.
• In order to actually write the alert, one would have to implement their own SyncWriteAdapter (I think?), so I made a helper function to write the alert down.
• In the case of writing the alert, I found that the alert is actually not present; I fixed this by returning immediately once we set the alert. This is because:
  • We get error, so we set the alert, continue the loop.
  • We take the alert, then discard it since its not Sending().
  • We get an error in read_tls so we return.