internal/test: allow images to be +/- 1 RGB value per pixel

This provides robustness when the rendering engine
behaves (ever so) slightly differently.

Fixes fyne-io#4573 (I think, I can't be entirely sure
because my tests still fail because of fyne-io#4572)

While we're here, let gofumpt add an octal marker to 0o755.

internal/test/util.go

@@ -10,7 +10,6 @@ import (
 	"path/filepath"
 	"testing"
 
-	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
@@ -44,13 +43,38 @@ func AssertImageMatches(t *testing.T, masterFilename string, img image.Image, ms
 	if len(msgAndArgs) > 0 {
 		msg = fmt.Sprintf(msgAndArgs[0].(string)+"\n", msgAndArgs[1:]...)
 	}
-	if !assert.Equal(t, masterPix, capturePix, "%sImage did not match master. Actual image written to file://%s.", msg, failedPath) {
+	if !pixCloseEnough(masterPix, capturePix) {
+		t.Logf("%sImage did not match master. Actual image written to file://%s.", msg, failedPath)
 		require.NoError(t, writeImage(failedPath, img))
 		return false
 	}
 	return true
 }
 
+// pixCloseEnough reports whether a and b vary by at most 1 at any given offset.
+func pixCloseEnough(a, b []uint8) bool {
+	if len(a) != len(b) {
+		return false
+	}
+
+	for i, v := range a {
+		w := b[i]
+		if v == w {
+			continue
+		}
+		// Allow a delta of 1 for rendering variation.
+		delta := int(v) - int(w) // use int to avoid overflow
+		if delta < 0 {
+			delta *= -1
+		}
+		if delta > 1 {
+			return false
+		}
+	}
+
+	return true
+}
+
 // NewCheckedImage returns a new black/white checked image with the specified size
 // and the specified amount of horizontal and vertical tiles.
 func NewCheckedImage(w, h, hTiles, vTiles int) image.Image {
@@ -83,7 +107,7 @@ func pixelsForImage(t *testing.T, img image.Image) []uint8 {
 }
 
 func writeImage(path string, img image.Image) error {
-	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return err
 	}
 	f, err := os.Create(path)