Split regression extracter and runner (#1451)

A lot of new sources of regression tests may come and it's important to
make debug simpler.

Now regression-extract.js downloads and write svg files into
test/regression-fixtures. regression.js run each svg in this folder.
Mismatched svg diff is written into test/regression-diffs.

test/regression-extract.js

@@ -0,0 +1,57 @@
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const util = require('util');
+const zlib = require('zlib');
+const stream = require('stream');
+const fetch = require('node-fetch');
+const tarStream = require('tar-stream');
+
+const pipeline = util.promisify(stream.pipeline);
+
+const extractTarGz = async (url, baseDir, include) => {
+  const extract = tarStream.extract();
+  extract.on('entry', async (header, stream, next) => {
+    try {
+      if (include == null || include.test(header.name)) {
+        if (header.name.endsWith('.svg')) {
+          const file = path.join(baseDir, header.name);
+          await fs.promises.mkdir(path.dirname(file), { recursive: true });
+          await pipeline(stream, fs.createWriteStream(file));
+        }
+        if (header.name.endsWith('.svgz')) {
+          // .svgz -> .svg
+          const file = path.join(baseDir, header.name.slice(0, -1));
+          await fs.promises.mkdir(path.dirname(file), { recursive: true });
+          await pipeline(
+            stream,
+            zlib.createGunzip(),
+            fs.createWriteStream(file)
+          );
+        }
+      }
+    } catch (error) {
+      console.error(error);
+      process.exit(1);
+    }
+    stream.resume();
+    next();
+  });
+  const response = await fetch(url);
+  await pipeline(response.body, extract);
+};
+
+(async () => {
+  try {
+    console.info('Download W3C SVG 1.1 Test Suite and extract svg files');
+    await extractTarGz(
+      'https://www.w3.org/Graphics/SVG/Test/20110816/archives/W3C_SVG_11_TestSuite.tar.gz',
+      path.join(__dirname, 'regression-fixtures', 'w3c-svg-11-test-suite'),
+      /^svg\//
+    );
+  } catch (error) {
+    console.error(error);
+    process.exit(1);
+  }
+})();