GroupByTests.cs: Added test case from NH-2560 (working), and two non-working variations reported as NH-3026 and NH-3027.
GroupByTests.cs: Added test case from NH-2560 (working), and two non-working variations reported as NH-3026 and NH-3027.

fredericDelaporte/nhibernate-core,nkreipke/nhibernate-core,RogerKratz/nhibernate-core,ManufacturingIntelligence/nhibernate-core,ngbrown/nhibernate-core,livioc/nhibernate-core,ngbrown/nhibernate-core,lnu/nhibernate-core,alobakov/nhibernate-core,gliljas/nhibernate-core,alobakov/nhibernate-core,RogerKratz/nhibernate-core,gliljas/nhibernate-core,ManufacturingIntelligence/nhibernate-core,nhibernate/nhibernate-core,fredericDelaporte/nhibernate-core,nkreipke/nhibernate-core,fredericDelaporte/nhibernate-core,lnu/nhibernate-core,livioc/nhibernate-core,gliljas/nhibernate-core,nhibernate/nhibernate-core,hazzik/nhibernate-core,hazzik/nhibernate-core,hazzik/nhibernate-core,fredericDelaporte/nhibernate-core,hazzik/nhibernate-core,nkreipke/nhibernate-core,ManufacturingIntelligence/nhibernate-core,lnu/nhibernate-core,nhibernate/nhibernate-core,RogerKratz/nhibernate-core,gliljas/nhibernate-core,nhibernate/nhibernate-core,ngbrown/nhibernate-core,livioc/nhibernate-core,RogerKratz/nhibernate-core,alobakov/nhibernate-core