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/bindKey.js

@@ -0,0 +1,66 @@
+import assert from 'assert';
+import { slice } from './utils.js';
+import bindKey from '../bindKey.js';
+
+describe('bindKey', function() {
+  it('should work when the target function is overwritten', function() {
+    var object = {
+      'user': 'fred',
+      'greet': function(greeting) {
+        return this.user + ' says: ' + greeting;
+      }
+    };
+
+    var bound = bindKey(object, 'greet', 'hi');
+    assert.strictEqual(bound(), 'fred says: hi');
+
+    object.greet = function(greeting) {
+      return this.user + ' says: ' + greeting + '!';
+    };
+
+    assert.strictEqual(bound(), 'fred says: hi!');
+  });
+
+  it('should support placeholders', function() {
+    var object = {
+      'fn': function() {
+        return slice.call(arguments);
+      }
+    };
+
+    var ph = bindKey.placeholder,
+        bound = bindKey(object, 'fn', ph, 'b', ph);
+
+    assert.deepStrictEqual(bound('a', 'c'), ['a', 'b', 'c']);
+    assert.deepStrictEqual(bound('a'), ['a', 'b', undefined]);
+    assert.deepStrictEqual(bound('a', 'c', 'd'), ['a', 'b', 'c', 'd']);
+    assert.deepStrictEqual(bound(), [undefined, 'b', undefined]);
+  });
+
+  it('should use `_.placeholder` when set', function() {
+    var object = {
+      'fn': function() {
+        return slice.call(arguments);
+      }
+    };
+
+    var _ph = _.placeholder = {},
+        ph = bindKey.placeholder,
+        bound = bindKey(object, 'fn', _ph, 'b', ph);
+
+    assert.deepEqual(bound('a', 'c'), ['a', 'b', ph, 'c']);
+    delete _.placeholder;
+  });
+
+  it('should ensure `new bound` is an instance of `object[key]`', function() {
+    function Foo(value) {
+      return value && object;
+    }
+
+    var object = { 'Foo': Foo },
+        bound = bindKey(object, 'Foo');
+
+    assert.ok(new bound instanceof Foo);
+    assert.strictEqual(new bound(true), object);
+  });
+});