Improve the subsystem test schema test coverage

Currently the way to enable the AbstractSubsystemBaseTest testSchema() and testSchemaOfSubsystemTemplates() tests is to override getSubsystemXsdPath() and getSubsystemTemplatePaths().

Rather than making it explicit to turn on, it should be explicit to turn off.

Also the current way of doing this uses Assume.assumeTrue() to check if a test has provided a schema file, which provides a lot of ignored test noise in the test output. If the xsd should not be tested, methods should instead override testSchema() or testSchemaOfSubsystemTemplates() and provide an empty implementation with a comment saying why it is not important.