NEW: Add status code comment to testsession/start output.

testsession/start now includes a comment of the form <!-- SUCCESS: DBNAME=DatabaseName -->,
which can be used by behat and other consumers to validate that the tesession was actually
started.

It's included the database name in its output, which is a small piece of information
disclosure, but not a big deal compared to the generally dev-only nature of this module.

It requires a fix to Cookie::set(), to ensure that set cookies also apperar immediately
in $_COOKIE.  Otherwise the call to DB::get_alternative_database_name() after it is set
won't return a value.

Requires https://github.com/silverstripe/silverstripe-framework/pull/2068