cmpimg: don't report errors in helper

Author: kortschak

cmpimg/checkplot.go

@@ -30,6 +30,8 @@ func goldenPath(path string) string {
 // For image.Image formats, a base64 encoded png representation is output to
 // the testing log when a difference is identified.
 func CheckPlot(ExampleFunc func(), t *testing.T, filenames ...string) {
+	t.Helper()
+
 	paths := make([]string, len(filenames))
 	for i, fn := range filenames {
 		paths[i] = filepath.Join("testdata", fn)