Convert PageCacheTest web tests to browser tests for page_cache module

Updated Title

This patch will fail as a couple of helper methods assertNoCacheTag and others are missing in AssertPageCacheContextsAndTagsTrait which are required for this test.

Also,
BTB drupalGetHeader function is calling getResponseHeader of Mink Session class which internally changing the string to lowercase which making couple of tests fail.

    public function getResponseHeader($name)
    {
        $headers = $this->driver->getResponseHeaders();

        $name = strtolower($name);
        $headers = array_change_key_case($headers, CASE_LOWER);

        if (!isset($headers[$name])) {
            return null;
        }

        return is_array($headers[$name]) ? $headers[$name][0] : $headers[$name];
    }

How about overriding this method in BTB?

I'll continue it tomorrow.
//Naveen