Detect the 'partition' token during filter element parsing

This adds support for a new PartitionFilter at nunit/nunit#4392

Author: Plasma

src/NUnitEngine/nunit.engine/Services/TestSelectionParser.cs

@@ -126,6 +126,7 @@ public string ParseFilterElement()
                 case "name":
                 case "test":
                 case "namespace":
+                case "partition":
                     Token op = lhs.Text == "id"
                         ? Expect(EQ_OPS)
                         : Expect(REL_OPS);