You did not fully specify which type of property data types should be included. You should make much more properties in the random tests of the following types. I added remarks because you haven't specified the following:

• What is a "List" in this kata? (System.Collections.List, System.Collections.Generic.List<>, System.Collections.Generic.IList<>, ...)
• What is considered an "Implementation" of an interface? (classes, other interfaces, the interface itself, ...)

Properties that should be ignored by user:

• List<object>
• IList<ISearchable> (or is IList also ok?)
• ISearchable[]

Properties that should be included by user:

• List<SomeClassThatImplementsISearchable>
• List<SomeInterfaceThatIsDerivedFromISearchable> (or is a derived interface not considered an impl of the interface?)
• List<SomeClassThatImplementsSomeInterfaceThatIsDerivedFromISearchable>
• List<ISearchable> (or is the interface not considered an impl of itself?)