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-constructor.js

@@ -0,0 +1,39 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { empties, stubTrue, isNpm, lodashBizarro } from './utils.js';
+
+describe('lodash constructor', function() {
+  var values = empties.concat(true, 1, 'a'),
+      expected = lodashStable.map(values, stubTrue);
+
+  it('should create a new instance when called without the `new` operator', function() {
+    var actual = lodashStable.map(values, function(value) {
+      return _(value) instanceof _;
+    });
+
+    assert.deepEqual(actual, expected);
+  });
+
+  it('should return the given `lodash` instances', function() {
+    var actual = lodashStable.map(values, function(value) {
+      var wrapped = _(value);
+      return _(wrapped) === wrapped;
+    });
+
+    assert.deepEqual(actual, expected);
+  });
+
+  it('should convert foreign wrapped values to `lodash` instances', function() {
+    if (!isNpm && lodashBizarro) {
+      var actual = lodashStable.map(values, function(value) {
+        var wrapped = _(lodashBizarro(value)),
+            unwrapped = wrapped.value();
+
+        return wrapped instanceof _ &&
+          ((unwrapped === value) || (unwrapped !== unwrapped && value !== value));
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    }
+  });
+});