Query: GroupBy add tests when grouping key is complex & aggregate is applied

[smitpatel]

See #12476

[ntierontime]

EF.Functions.DateDiffDay is not translated properly when grouped by.

Personally, this is a standard usage.
The EF Query is translated to following sql:

SELECT DATEPART(year, [t].[RecordDate]) AS [Year], DATEPART(month, [t].[RecordDate]) AS [Month], @___DateTimeNow_1 AS [AverageVirtualAge]
FROM [dbo].[TouchLog] AS [t]
GROUP BY DATEPART(year, [t].[RecordDate]), DATEPART(month, [t].[RecordDate])

    public partial class TouchLog
    {
        public long TouchLogId { get; set; }
        public DateTime? RecordDate { get; set; }
    }

            DateTime _DateTimeNow = DateTime.Now;

            var report = from t in _dbContext.TouchLog
                    select new { Year = t.RecordDate.Value.Year, Month = t.RecordDate.Value.Month, RecordDate = t.RecordDate, VirtualAge = EF.Functions.DateDiffDay(t.RecordDate, _DateTimeNow) } into tNew
                    group new { t = tNew } by new { Year = tNew.Year, Month = tNew.Month } into tG
                    let _AverageVirtualAge = tG.Average(p => (double?)EF.Functions.DateDiffDay(p.t.RecordDate, _DateTimeNow))
                    //let _AverageVirtualAge = tG.Average(p => (double?)p.t.VirtualAge)
                    //let _AverageVirtualAge = tG.Select(p => (double?)p.t.VirtualAge).Average()
                    select new 
                    {
                        Year = tG.Key.Year,
                        Month = tG.Key.Month,
                        AverageVirtualAge = _AverageVirtualAge
                    } into vD6
                    select vD6;