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/isEmpty.js

@@ -0,0 +1,119 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+
+import {
+  empties,
+  stubTrue,
+  slice,
+  symbol,
+  args,
+  push,
+  arrayProto,
+  realm,
+  MAX_SAFE_INTEGER,
+} from './utils.js';
+
+import isEmpty from '../isEmpty.js';
+
+describe('isEmpty', function() {
+  it('should return `true` for empty values', function() {
+    var expected = lodashStable.map(empties, stubTrue),
+        actual = lodashStable.map(empties, isEmpty);
+
+    assert.deepStrictEqual(actual, expected);
+
+    assert.strictEqual(isEmpty(true), true);
+    assert.strictEqual(isEmpty(slice), true);
+    assert.strictEqual(isEmpty(1), true);
+    assert.strictEqual(isEmpty(NaN), true);
+    assert.strictEqual(isEmpty(/x/), true);
+    assert.strictEqual(isEmpty(symbol), true);
+    assert.strictEqual(isEmpty(), true);
+
+    if (Buffer) {
+      assert.strictEqual(isEmpty(new Buffer(0)), true);
+      assert.strictEqual(isEmpty(new Buffer(1)), false);
+    }
+  });
+
+  it('should return `false` for non-empty values', function() {
+    assert.strictEqual(isEmpty([0]), false);
+    assert.strictEqual(isEmpty({ 'a': 0 }), false);
+    assert.strictEqual(isEmpty('a'), false);
+  });
+
+  it('should work with an object that has a `length` property', function() {
+    assert.strictEqual(isEmpty({ 'length': 0 }), false);
+  });
+
+  it('should work with `arguments` objects', function() {
+    assert.strictEqual(isEmpty(args), false);
+  });
+
+  it('should work with prototype objects', function() {
+    function Foo() {}
+    Foo.prototype = { 'constructor': Foo };
+
+    assert.strictEqual(isEmpty(Foo.prototype), true);
+
+    Foo.prototype.a = 1;
+    assert.strictEqual(isEmpty(Foo.prototype), false);
+  });
+
+  it('should work with jQuery/MooTools DOM query collections', function() {
+    function Foo(elements) {
+      push.apply(this, elements);
+    }
+    Foo.prototype = { 'length': 0, 'splice': arrayProto.splice };
+
+    assert.strictEqual(isEmpty(new Foo([])), true);
+  });
+
+  it('should work with maps', function() {
+    if (Map) {
+      lodashStable.each([new Map, realm.map], function(map) {
+        assert.strictEqual(isEmpty(map), true);
+        map.set('a', 1);
+        assert.strictEqual(isEmpty(map), false);
+        map.clear();
+      });
+    }
+  });
+
+  it('should work with sets', function() {
+    if (Set) {
+      lodashStable.each([new Set, realm.set], function(set) {
+        assert.strictEqual(isEmpty(set), true);
+        set.add(1);
+        assert.strictEqual(isEmpty(set), false);
+        set.clear();
+      });
+    }
+  });
+
+  it('should not treat objects with negative lengths as array-like', function() {
+    function Foo() {}
+    Foo.prototype.length = -1;
+
+    assert.strictEqual(isEmpty(new Foo), true);
+  });
+
+  it('should not treat objects with lengths larger than `MAX_SAFE_INTEGER` as array-like', function() {
+    function Foo() {}
+    Foo.prototype.length = MAX_SAFE_INTEGER + 1;
+
+    assert.strictEqual(isEmpty(new Foo), true);
+  });
+
+  it('should not treat objects with non-number lengths as array-like', function() {
+    assert.strictEqual(isEmpty({ 'length': '0' }), false);
+  });
+
+  it('should return an unwrapped value when implicitly chaining', function() {
+    assert.strictEqual(_({}).isEmpty(), true);
+  });
+
+  it('should return a wrapped value when explicitly chaining', function() {
+    assert.ok(_({}).chain().isEmpty() instanceof _);
+  });
+});