Added support for Action delegates in TestCase class

This could probably address #16.

Please notice that, in the files changed, I didn't have to return a different `ITestCommand` instance as [suggested in the description](https://github.com/GreanTech/Exude/issues/16#issue-29510346).

The following tests passed by only adapting `Action` to `Action<object>` in the `TestCase` class:

```
[FirstClassTests]
public static IEnumerable<ITestCase> YieldFirstClassTests()
{
    yield return new TestCase(() => Assert.Equal(1, 1));
    yield return new TestCase(() => Assert.Equal(2, 2));
    yield return new TestCase(() => Assert.Equal(3, 3));
}
```