tests: added golden files use and generation example

Author: Dmitry Sinyavskiy

testing/golden-files/golden.go

@@ -0,0 +1,33 @@
+package somepacakge_test
+
+import (
+	"bytes"
+	"flag"
+	"io/ioutil"
+	"path/filepath"
+	"testing"
+)
+
+// This snippet shows how to create and use golden-files. They helps us to test complex output without manually hardcoding it
+// Very scalable way to test complex structures (write a String() method)
+//
+// = How to use golden files? =
+// * generate golden files by running test with `-update-golden` flag (`go test -update-golden`)
+// * check manually generated data in a files and if it's correct then commit it.
+
+var updateGolden = flag.Bool("update-golden", false, "update golden files")
+
+func TestAdd(t *testing.T) {
+	// ... table (probably!)
+	for _, tc := range сases {
+		actual := doSomething(tc)
+		golden := filepath.Join("test-fixtures", tc.Name+".golden")
+		if *updateGolden {
+			ioutil.WriteFile(golden, actual, 0644)
+		}
+		expected, _ := ioutil.ReadFile(golden)
+		if !bytes.Equal(actual, expected) {
+			// FAIL!
+		}
+	}
+}