ExpectFile as a helper to improve test tracing (#54)

Author: alexbarotur

autogold.go

@@ -47,6 +47,7 @@ func update() bool {
 // If the input value is of type Raw, its contents will be directly used instead of the value being
 // formatted as a Go literal.
 func ExpectFile(t *testing.T, got interface{}, opts ...Option) {
+	t.Helper()
 	dir := testdataDir(opts)
 	fileName := testName(t, opts)
 	outFile := filepath.Join(dir, fileName+".golden")