Fix bug in drb tests if hostname does not resolve to a local IP
3 of the drb tests leave the hostname part of the druby URI empty instead of using localhost. When a hostname is not provided, ruby uses the system hostname, which usually resolves to a local IP address, but not always. This causes the tests to fail if the system hostname doesn't resolve to a local IP address. The tests also appear to hang, since the connection timeout is 100 seconds.
The attached patch fixes this case by using localhost as the hostname in the 3 tests that did not already do that, making them consistent with the other tests.