Add initial test files from lodash v4. (#4172)

* Install test dependencies.

* Add initial test files.

These files were created using a simplistic AST manipulator using `recast` to
preserve formatting. There's bound to be a huge chunk of errors, but this serves
as a good start. QUnit was replaced with Mocha, with ES2015 imports running via
`esm`.

As far as possible, QUnit-specific syntax has been replaced with Mocha's
`describe` and `it`, while the native Node.js `assert` module is used for
assertions. Files in the `test` directory ending in `.test.js` will be treated
as test files.

* Add initial passing files to test run.

test/meanBy.js

@@ -0,0 +1,31 @@
+import assert from 'assert';
+import { slice } from './utils.js';
+import meanBy from '../meanBy.js';
+
+describe('meanBy', function() {
+  var objects = [{ 'a': 2 }, { 'a': 3 }, { 'a': 1 }];
+
+  it('should work with an `iteratee`', function() {
+    var actual = meanBy(objects, function(object) {
+      return object.a;
+    });
+
+    assert.deepStrictEqual(actual, 2);
+  });
+
+  it('should provide correct `iteratee` arguments', function() {
+    var args;
+
+    meanBy(objects, function() {
+      args || (args = slice.call(arguments));
+    });
+
+    assert.deepStrictEqual(args, [{ 'a': 2 }]);
+  });
+
+  it('should work with `_.property` shorthands', function() {
+    var arrays = [[2], [3], [1]];
+    assert.strictEqual(meanBy(arrays, 0), 2);
+    assert.strictEqual(meanBy(objects, 'a'), 2);
+  });
+});