Roslyn tests for DynamicLinqWriterVisitor results

[zspitz]

Pending #82 we're changing the output of DynamicLinqWriterVisitor from just being an expression, to actual code that can be copied/pasted. We should implement Roslyn tests to verify that these actually work.

Related #66.

[zspitz]

One possible issue is how we render object instances with no ToString override. For example, an instance of System.Random becomes #Random.