Ensure that golden_test fails if it doesn't find any goldens. (#6)

diff --git a/goldens/golden_test.go b/goldens/golden_test.go
index 0d28e73..b0c07f0 100644
--- a/goldens/golden_test.go
+++ b/goldens/golden_test.go
@@ -45,6 +45,10 @@
 		}
 	}
 
+	if len(tcs) == 0 {
+		t.Fatalf("Did not find any golden files.")
+	}
+
 	needsRegen := make(chan string, len(tcs))
 	t.Run("group", func(t *testing.T) {
 		for _, tc := range tcs {