Improve test coverage

[snowleopard]

Despite having 640+ QuickCheck properties, test coverage of some modules is not great:

module	Top Level Definitions	Alternatives	Expressions
	%	%	%
Algebra.Graph	70%	93%	91%
Algebra.Graph.AdjacencyIntMap	100%	100%	100%
Algebra.Graph.AdjacencyIntMap.Internal	73%	100%	95%
Algebra.Graph.AdjacencyMap	100%	100%	99%
Algebra.Graph.AdjacencyMap.Internal	73%	100%	95%
Algebra.Graph.Class	50%	40%	28%
Algebra.Graph.Export	72%	100%	89%
Algebra.Graph.Export.Dot	42%	100%	92%
Algebra.Graph.Fold	63%	100%	86%
Algebra.Graph.HigherKinded.Class	-	-	-
Algebra.Graph.Internal	-	-	-
Algebra.Graph.NonEmpty	-	-	-
Algebra.Graph.Relation	100%	100%	100%
Algebra.Graph.Relation.Internal	77%	100%	91%
Algebra.Graph.Relation.InternalDerived	40%	-	84%
Algebra.Graph.Relation.Symmetric	66%	-	87%
Algebra.Graph.ToGraph	100%	-	100%
Data.Graph.Typed	88%	-	94%
Program Coverage Total	76%	92%	90%

Let's improve this.