Consider generating model classes to improve test coverage

[AndriySvyryd]

This would allow to reuse the same test code for different permutations of

• Virtual/non-virtual/Lazy loading/Proxies
• Auto/backing fields/indexer/no property
• public/private field/property
• public setter/private setter/readonly/constructor only
• Snapshot/Changed/Full notifications
• new/override/explicit implementation properties
• nullable/non-nullable properties and navigations
• CLR default/custom default/calculated property
• Reference and collection navigations non-initialized/initialized in constructor
• Navigations to exact type/base/interface
• Custom Equals/GetHashCode
• Collection types (IEnumerable/read-only/immutable/etc.)

[ajcvickers]

Time-box to 1 week for now, and let's see where we get.

[AndriySvyryd]

Design proposal:

• Tests are written against interfaces that are implemented implicitly or explicitly by DbContext and model classes
• The entity class generator takes an IModel created with a sample implementation as an input and outputs the requested DbContext and model classes implementations in .cs files