Was relying on cookie to set on NEXT request, which was too late
since some of the following init logic relied on DB queries.
This happened to work if your non-test DB was already set up,
but failed on fresh checkouts.
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.
