Backoff and Randomness additions for use in client utilities

Updated String to use int, and int not to panic, and panic to test panic
Format fixes.

pkg/util/rand/rand_test.go

@@ -37,6 +37,27 @@ func TestString(t *testing.T) {
 	}
 }
 
+// Confirm that panic occurs on invalid input.
+func TestRangePanic(t *testing.T) {
+	defer func() {
+		if err := recover(); err == nil {
+			t.Errorf("Panic didn't occur!")
+		}
+	}()
+	// Should result in an error...
+	Intn(0)
+}
+
+func TestIntn(t *testing.T) {
+	// 0 is invalid.
+	for _, max := range []int{1, 2, 10, 123} {
+		inrange := Intn(max)
+		if inrange < 0 || inrange > max {
+			t.Errorf("%v out of range (0,%v)", inrange, max)
+		}
+	}
+}
+
 func TestPerm(t *testing.T) {
 	Seed(5)
 	rand.Seed(5)