Add XSD validation of subsystem's XML

add methods in AbstractSubsystemBaseTest to validate the
subsystem's XML (either from the tests or from the templates
used to build the server configuration) against a given XSD schema

Subsystems need to override getSubsystemXsdPath() and
getSubsystemTemplates() to active the validation tests.

* validate IO susbystem's XML as an example
Good idea. I went a step further and added a 

````
protected Properties getResolvedProperties() {
    return new Properties();
}
```

that are the _only_ properties used to resolve the expressions as I have removed the use of `SystemPropertyResolver.INSTANCE`. I prefer to avoid dealing with system properties in the tests if I can. I've been bitten too many times by a test forgetting to remove a sys prop and making it available for all subsequent tests :)

I use this `getResolvedProperties()` method in Undertow for example: https://github.com/jmesnil/wildfly/commit/f33612b6043a193252e40352751687fb1436d34e