cubicweb #550162 automatic web tests: default content type validator should be None [resolved]

instead of SAXOnly or something

turning it into a list/tuple would make it more easily extensible

prioritynormal
typebug
done in<not specified>
load0.100
load left0.000
closed by#5d1b8fc9cb98 don't use html/dtd validator as default validator for non html views. Closes #550162