behat: add test for --url global parameter
set $_SERVER['SERVER_PORT']
behat: install WP with custom URL
append port to $_SERVER['HTTP_HOST']
don't set $_SERVER['REQUEST_URL']

not used by WordPress and probably bogus
behat: set custom port only in that specific test, since multisite installs don't like it
Merge pull request #556 from wp-cli/custom-port

`wp core install` ignores custom port