Add testIterator for use in testing

graph/iterator/iterator_test.go

@@ -0,0 +1,29 @@
+package iterator
+
+import (
+	"github.com/google/cayley/graph"
+)
+
+// A testing iterator that returns the given values for Next() and Err().
+type testIterator struct {
+	*Fixed
+
+	NextVal bool
+	ErrVal  error
+}
+
+func newTestIterator(next bool, err error) graph.Iterator {
+	return &testIterator{
+		Fixed:   NewFixed(Identity),
+		NextVal: next,
+		ErrVal:  err,
+	}
+}
+
+func (it *testIterator) Next() bool {
+	return it.NextVal
+}
+
+func (it *testIterator) Err() error {
+	return it.ErrVal
+}