Add fuzzer for map generator testing

Adds a fuzzer that iterates through map generation settings combinations
to either discover crashes or measure the frequency of rejected maps.

All generated maps are in-memory only and discarded immediately after
generation.

Running the fuzzer on anything other than a small combination of
settings is time expensive. It is added only as a utility for manual
invocation and is not integrated into the regular set of tests.

OpenRA.Mods.Common/MapGenerator/MapGeneratorSettings.cs

@@ -296,6 +296,23 @@ namespace OpenRA.Mods.Common.MapGenerator
 					return Default;
 				}
 			}
+
+			public bool IsFreeform()
+			{
+				switch (Ui)
+				{
+					case UiType.Hidden:
+					case UiType.DropDown:
+					case UiType.Checkbox:
+						return false;
+					case UiType.Integer:
+					case UiType.Float:
+					case UiType.String:
+						return true;
+					default:
+						throw new InvalidOperationException("Bad UiType");
+				}
+			}
 		}
 
 		public readonly IReadOnlyList<Option> Options;