Demonstrate XInclude work - and fail

[dubinsky]

This pull request does not affect production code at all; it:

• adds a test dependency on Xerces;
• adjust tests that assume that JDK-built-in Xerces is the only Xerces on the classpath;
• adds tests demonstrating that XInclude works;
• adds tests demonstrating when XInclude fails miserably because of the old and never fixed Xerces bug...

[SethTisue]

I don't have any expertise on the specifics here, but I think you should go ahead and merge it.

[dubinsky]

I don't have any expertise on the specifics here, but I think you should go ahead and merge it.

Thank you!
Maybe you have some ideas on how we can encourage the Xerces team to fix a 20-years old bug? ;)