Move test classes into a separate source tree.

This also has the effect of excluding test classes from the Haxelib
distribution, which is probably a good idea.
Create Haxelib zip directly with no intermediate file structure.
Remove redundant classpath.

I'm not sure why this was ever here.
Add test case runner.
Fix test cases to compile in Haxe 3.
Fix XPathHxXml to compile in Haxe 3.
Allow test cases to compile and run on non-system platforms (JS, Flash).
Remove special case that ignores part of a test for Flash 9.

If this is broken on Flash 9 we need to know about it.