Fix assertEquals() in BP_Tests_URL class.

The test_bp_core_ajax_url() method assumes that the WP_TESTS_DOMAIN
constant is always the default 'example.org'.

When the WP_TESTS_DOMAIN is different, the assertEquals() method will fail.

This commit changes the hardcoded example.org checks to use get_site_url().
Merge pull request #1 from r-a-y/url

Fix assertEquals() in BP_Tests_URL class.