When we register a closure as file, the closure will be exectutd.
Otherwise we return what is registered.
This gives us a maximum level of flexibility. We can throw exceptions (e.g. FileNotFound) as
as test setup requires this, in simple cases we provide the yaml as a string.
read yaml rule is a very frequently used rule. But having the rule in the common rules
means we cannot register text or files to that rule, which makes it less handy to work
with yaml files in the tests.
