Throw for LINQ queries on unmapped entities

[bahusoid]

Fixes #1106

[bahusoid]

I simply skipped invalid tests in FunctionTests as they didn't really test anything (as those tests are simply based on the fact that query can be executed and dump results to output).

P.S. We need to do something with tests inside Linq folder.
As now all of them are executed under LinqReadonlyTestsContext . So it's problematic to add some non readonly test or test not based on Northwind database. And mixing non-readonly and readonly tests can lead to some vague issues (like missing sequence on firebird for unrelated tests).

[hazzik]

As I understand it the idea of LinqReadOnlyTextContext was exactly about that - running read only tests. So, IMO, non read only tests should go somewhere else.