Refactor backend SVG package and add tests (#26335)

Introduce a well-tested `svg.Normalize` function.
Make `RenderHTML` faster and more stable.

modules/svg/processor_test.go

@@ -0,0 +1,29 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package svg
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNormalize(t *testing.T) {
+	res := Normalize([]byte("foo"), 1)
+	assert.Equal(t, "foo", string(res))
+
+	res = Normalize([]byte(`<?xml version="1.0"?>
+<!--
+comment
+-->
+<svg xmlns = "...">content</svg>`), 1)
+	assert.Equal(t, `<svg width="1" height="1" class="svg">content</svg>`, string(res))
+
+	res = Normalize([]byte(`<svg
+width="100"
+class="svg-icon"
+>content</svg>`), 16)
+
+	assert.Equal(t, `<svg class="svg-icon" width="16" height="16">content</svg>`, string(res))
+}