Use a URL with a Host component in BrokerChannel tests.
The tests were using a broker URL of "test.broker" (i.e., a schema-less, host-less, relative path), and running assertions on the value of b.url.Path. This is strange, especially in tests regarding domain fronting, where we care about b.url.Host, not b.url.Path. This commit changes the broker URL to "http://test.broker" and changes tests to check b.url.Host. I also added an additional assertion for an empty b.Host in the non-domain-fronted case.