Migrate to jest (#1520)

Mocha doesn't have a lot of features provided by jest.
There is a great assertion library out of the box.
And the most cool feature is inline snapshots.
Mocha also hides errors which makes debugging a nightmare sometimes.

test/plugins/_index.test.js

@@ -0,0 +1,58 @@
+'use strict';
+
+const FS = require('fs');
+const PATH = require('path');
+const EOL = require('os').EOL;
+const regEOL = new RegExp(EOL, 'g');
+const regFilename = /^(.*)\.(\d+)\.svg$/;
+const { optimize } = require('../../lib/svgo.js');
+
+describe('plugins tests', function () {
+  FS.readdirSync(__dirname).forEach(function (file) {
+    var match = file.match(regFilename),
+      index,
+      name;
+
+    if (match) {
+      name = match[1];
+      index = match[2];
+
+      file = PATH.resolve(__dirname, file);
+
+      it(name + '.' + index, function () {
+        return readFile(file).then(function (data) {
+          // remove description
+          const items = normalize(data).split(/\s*===\s*/);
+          const test = items.length === 2 ? items[1] : items[0];
+          // extract test case
+          const [original, should, params] = test.split(/\s*@@@\s*/);
+          const plugin = {
+            name,
+            params: params ? JSON.parse(params) : {},
+          };
+          const result = optimize(original, {
+            path: file,
+            plugins: [plugin],
+            js2svg: { pretty: true },
+          });
+          expect(result.error).not.toEqual(expect.anything());
+          //FIXME: results.data has a '\n' at the end while it should not
+          expect(normalize(result.data)).toEqual(should);
+        });
+      });
+    }
+  });
+});
+
+function normalize(file) {
+  return file.trim().replace(regEOL, '\n');
+}
+
+function readFile(file) {
+  return new Promise(function (resolve, reject) {
+    FS.readFile(file, 'utf8', function (err, data) {
+      if (err) return reject(err);
+      resolve(data);
+    });
+  });
+}