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/lodash(...).push.js

@@ -0,0 +1,27 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { falsey, stubTrue } from './utils.js';
+
+describe('lodash(...).push', function() {
+  it('should append elements to `array`', function() {
+    var array = [1],
+        wrapped = _(array).push(2, 3),
+        actual = wrapped.value();
+
+    assert.strictEqual(actual, array);
+    assert.deepEqual(actual, [1, 2, 3]);
+  });
+
+  it('should accept falsey arguments', function() {
+    var expected = lodashStable.map(falsey, stubTrue);
+
+    var actual = lodashStable.map(falsey, function(value, index) {
+      try {
+        var result = index ? _(value).push(1).value() : _().push(1).value();
+        return lodashStable.eq(result, value);
+      } catch (e) {}
+    });
+
+    assert.deepEqual(actual, expected);
+  });
+});