[util][testutil] Add dump val as pretty json

[at15]

https://github.com/dyweb/gommon/blob/master/util/testutil/data.go only contains read but no write

Currently we only have

func WriteFixture(t *testing.T, path string, data []byte) {
	err := ioutil.WriteFile(path, data, 0664)
	if err != nil {
		t.Fatalf("can't write fixture %s: %v", path, err)
	}
}

Need

func SaveAsJson(t *testing.T, v interface{}, file string) {
	b, err := json.Marshal(v)
	if err != nil {
		t.Fatalf("failed to encode as json %v", v)
		return
	}
	if err := ioutil.WriteFile(file, b, 0664); err != nil {
		t.Fatalf("failed to save file %s: %v", file, err)
		return
	}
}

func SaveAsPrettyJson(t *testing.T, v interface{}, file string) {
	b, err := json.MarshalIndent(v, "", "  ")
	if err != nil {
		t.Fatalf("failed to encode as json %v", v)
		return
	}
	if err := ioutil.WriteFile(file, b, 0664); err != nil {
		t.Fatalf("failed to save file %s: %v", file, err)
		return
	}
	t.Logf("saved json to %s", file)
}

func SaveAsPrettyJsonf(t *testing.T, v interface{}, format string, args ...interface{}) {
	file := fmt.Sprintf(format, args...)
	b, err := json.MarshalIndent(v, "", "  ")
	if err != nil {
		t.Fatalf("failed to encode as json %v", v)
		return
	}
	if err := ioutil.WriteFile(file, b, 0664); err != nil {
		t.Fatalf("failed to save file %s: %v", file, err)
		return
	}
	t.Logf("saved json to %s", file)
}

[at15]

already in #90