Allow single type as return of [DynamicData] sources

[Evangelink]

Summary

Simplify even further allowed type for the data souce methods.

Background and Motivation

Data sources provided for [DynamicData] have been simplified to allow IEnumerable<ITuple> in addition to IEnumerable<object[]> and we should go even further by allowing T to be any type (e.g. IEnumerable<int>, IEnumerable<MyType>...). As you may still want to use a source despite having a single parameter in your test method as you benefit from unfolding/expanding.

Proposed Feature

Allow any IEnumerable<T> type.