gen-test: add readable text diff output

test plan:
 * go test -run '^Test_GeneratorTestSuite$' \
	-testify.m '^(TestGenerationFiles)$' \
	github.com/go-enry/go-enry/v2/internal/code-generator/generator

internal/code-generator/generator/diff/diff.go

@@ -0,0 +1,58 @@
+package diff
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+func writeTempFile(dir, prefix string, data []byte) (string, error) {
+	file, err := os.CreateTemp(dir, prefix)
+	if err != nil {
+		return "", err
+	}
+
+	_, err = file.Write(data)
+	if err1 := file.Close(); err == nil {
+		err = err1
+	}
+	if err != nil {
+		os.Remove(file.Name())
+		return "", err
+	}
+	return file.Name(), nil
+}
+
+// Diff returns a human-readable description of the differences between s1 and s2.
+func Diff(b1, b2 []byte) (string, error) {
+	if bytes.Equal(b1, b2) {
+		return "", nil
+	}
+
+	cmd := "diff"
+	if _, err := exec.LookPath(cmd); err != nil {
+		return "", fmt.Errorf("diff command unavailable\nold: %q\nnew: %q", b1, b2)
+	}
+
+	f1, err := writeTempFile("", "gen_test", b1)
+	if err != nil {
+		return "", err
+	}
+	defer os.Remove(f1)
+
+	f2, err := writeTempFile("", "gen_test", b2)
+	if err != nil {
+		return "", err
+	}
+	defer os.Remove(f2)
+
+	data, err := exec.Command(cmd, "-u", f1, f2).CombinedOutput()
+	if len(data) > 0 { // diff exits with a non-zero status when the files don't match
+		err = nil
+	}
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}

internal/code-generator/generator/generator_test.go

@@ -2,6 +2,7 @@ package generator
 
 import (
 	"flag"
+	"fmt"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -9,6 +10,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/go-enry/go-enry/v2/internal/code-generator/generator/diff"
 	"github.com/go-enry/go-enry/v2/internal/tokenizer"
 
 	"github.com/stretchr/testify/assert"
@@ -321,11 +323,17 @@ func (s *GeneratorTestSuite) TestGenerationFiles() {
 
 		expected := normalizeSpaces(string(gold))
 		actual := normalizeSpaces(string(out))
-		assert.Equal(s.T(), expected, actual, "Test %s", test.name)
+		// this produces large unreadable output, so we do it 'manually' instead
+		// assert.Equal(s.T(), expected, actual, "Test %s", test.name)
 		if expected != actual {
-			s.T().Logf("%s generated is different from %q", test.name, test.wantOut)
-			s.T().Logf("Expected %q", expected[:400])
-			s.T().Logf("Actual %q", actual[:400])
+			assert.Fail(s.T(), fmt.Sprintf("%s output is different from %q", test.name, test.wantOut))
+			diff, err := diff.Diff(gold, out)
+			if err != nil {
+				s.T().Logf("Failed produce a diff between expected and actual: %s", err.Error())
+				s.T().Logf("Expected %q", expected[:400])
+				s.T().Logf("Actual %q", actual[:400])
+			}
+			s.T().Logf("\n%s", diff)
 		}
 
 	}