Integrate unit-tests from the FHIR spec for the validator and snapshot generator

[ewoutkramer]

To ensure consistencies between the .NET and Java implementations, we should run our snapshot generator on the set of differentials that Grahame has created, so we can compare outputs.

Same holds true for the test-instances and expected validation messages - though these are harder to compare.

[wmrutten]

Also see #539

[mmsmits]

Done this for the validator, see #539 for snapshot generator.
Closed as duplicate