Improve ServerRestartRequiredTestCase.testSocketBinding() when used with ignore-unused-configuration

Currently this test adds an unused socket-binding-group that it assumes will not be ignored on the slave. Since the SBG is unused, it is ignored.

This test should just actually bind something to use the SBG, so the slave picks it up.

For now, the testsuite host-slave.xml has the bogus reload-sockets added statically.