TestHTTPServer: Make listening an explicit operation that reports failure.

Use this to print the error message when listening fails, and switch to always
stack allocating TestHTTPServer instances for easier cleanup.

Change-Id: I63b2bd38963b66611dc08a5c322615d91a91e675
Reviewed-by: John Brooks <john.brooks@dereferenced.net>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>