Add check for statistics

[vbreuss]

After implementing Testably/Testably.Abstractions#476, add checks to validate statistical information.

Examples:

mockFileSystem.Statistics.File.Should()
    .HaveCalled(f => f.WriteAllText)
    .WithFirstParameter("path/to/file")
    .ExactlyOnce();

mockFileSystem.Statistics.Path.Should()
    .HaveAccessed(p => p.DirectorySeparatorChar)
    .Never();