Xml schema testing

[wouterj]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#5
License	MIT
Doc PR	symfony-cmf/symfony-cmf-docs#335

This PR is the continuation of #9

Instead of using helpers, I used annotations. This results in better error messages. It is also fully tested.

[dantleech]

looks good! I am OK for this to be merged - although as an aside I have been wondering if we should be creating a repository per testing feature - I quite like that approach with https://github.com/matthiasnoback/SymfonyConfigTest

[wouterj]

Yeah, I was wondering about that too. The Testing component was first created to bundle all testing tools used by cmf bundles. But now we want to be standalone, we should maybe seperate it.

What do you think of trying to do this for 1.2? We can do some sort of substree split, the same as used for the security component in the core.

[dbu]

+1 to merge this and think about a splitting or ideally subtree-splitting and necessary refactorings for 1.2

[wouterj]

OK, I'm merging this now, then we can test all our schema's and make sure they are correct