add working wallaby config

As discussed at wallabyjs/public#1490 (comment)

wallaby.js

@@ -0,0 +1,49 @@
+'use strict';
+
+module.exports = () => {
+  return {
+    files: [
+      'index.js', 'lib/**/*.js', 'test/setup.js',
+      {
+        pattern: 'test/node-unit/**/*.fixture.js',
+        instrument: false
+      }, {
+        pattern: 'test/unit/**/*.fixture.js',
+        instrument: false
+      }
+    ],
+    filesWithNoCoverageCalculated: ['test/**/*.fixture.js'],
+    tests: [
+      'test/unit/**/*.spec.js', 'test/node-unit/**/*.spec.js'
+    ],
+    env: {
+      type: 'node',
+      runner: 'node'
+    },
+    workers: {recycle: true},
+    testFramework: {type: 'mocha', path: __dirname},
+    setup (wallaby) {
+      // running mocha instance is not the same as mocha under test,
+      // running mocha is the project's source code mocha, mocha under test is instrumented version of the source code
+      const runningMocha = wallaby.testFramework;
+      runningMocha.timeout(200);
+      // to expose it/describe etc. on the mocha under test
+      const mochaUnderTest = new (require('./'))();
+      mochaUnderTest.suite.emit('pre-require', global, '', mochaUnderTest);
+      // to make test/node-unit/color.spec.js pass, we need to run mocha in the project's folder context
+      const childProcess = require('child_process');
+      const execFile = childProcess.execFile;
+      childProcess.execFile = function () {
+        let opts = arguments[2];
+        if (typeof opts === 'function') {
+          opts = {};
+          Array.prototype.splice.call(arguments, 2, 0, opts);
+        }
+        opts.cwd = wallaby.localProjectDir;
+        return execFile.apply(this, arguments);
+      };
+      require('./test/setup');
+    },
+    debug: true
+  };
+};