INFRA-124 - civicrm.settings.php - Softer define()s

INFRA-124 will require that we make some defines (e.g. CIVICRM_DSN) dynamic
when running multiple test processes.  This patch allows us to
inject/override those settings without fully parsing/understanding the
content of civicrm.settings.php.
INFRA-124 - CiviSeleniumTestCase - Load cookies from $this->settings->cookies

For INFRA-124, we'll need a way to distinguish requests that correspond to
different test-instances.  We'll do that by passing along a cookie.
Merge pull request #3723 from totten/master-phpunit-para

INFRA-124 - Facilitate parallel testing by making test environment more flexible