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/constant.js

@@ -0,0 +1,40 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { empties, _, falsey, stubTrue } from './utils.js';
+
+describe('constant', function() {
+  it('should create a function that returns `value`', function() {
+    var object = { 'a': 1 },
+        values = Array(2).concat(empties, true, 1, 'a'),
+        constant = _.constant(object);
+
+    var results = lodashStable.map(values, function(value, index) {
+      if (index < 2) {
+        return index ? constant.call({}) : constant();
+      }
+      return constant(value);
+    });
+
+    assert.ok(lodashStable.every(results, function(result) {
+      return result === object;
+    }));
+  });
+
+  it('should work with falsey values', function() {
+    var expected = lodashStable.map(falsey, stubTrue);
+
+    var actual = lodashStable.map(falsey, function(value, index) {
+      var constant = index ? _.constant(value) : _.constant(),
+          result = constant();
+
+      return (result === value) || (result !== result && value !== value);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should return a wrapped value when chaining', function() {
+    var wrapped = _(true).constant();
+    assert.ok(wrapped instanceof _);
+  });
+});