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/size.test.js

@@ -0,0 +1,75 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { falsey, stubZero, args, push, arrayProto, realm, MAX_SAFE_INTEGER } from './utils.js';
+import size from '../size.js';
+
+describe('size', function() {
+  var array = [1, 2, 3];
+
+  it('should return the number of own enumerable string keyed properties of an object', function() {
+    assert.strictEqual(size({ 'one': 1, 'two': 2, 'three': 3 }), 3);
+  });
+
+  it('should return the length of an array', function() {
+    assert.strictEqual(size(array), 3);
+  });
+
+  it('should accept a falsey `object`', function() {
+    var expected = lodashStable.map(falsey, stubZero);
+
+    var actual = lodashStable.map(falsey, function(object, index) {
+      try {
+        return index ? size(object) : size();
+      } catch (e) {}
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should work with `arguments` objects', function() {
+    assert.strictEqual(size(args), 3);
+  });
+
+  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(size(new Foo(array)), 3);
+  });
+
+  it('should work with maps', function() {
+    if (Map) {
+      lodashStable.each([new Map, realm.map], function(map) {
+        map.set('a', 1);
+        map.set('b', 2);
+        assert.strictEqual(size(map), 2);
+        map.clear();
+      });
+    }
+  });
+
+  it('should work with sets', function() {
+    if (Set) {
+      lodashStable.each([new Set, realm.set], function(set) {
+        set.add(1);
+        set.add(2);
+        assert.strictEqual(size(set), 2);
+        set.clear();
+      });
+    }
+  });
+
+  it('should not treat objects with negative lengths as array-like', function() {
+    assert.strictEqual(size({ 'length': -1 }), 1);
+  });
+
+  it('should not treat objects with lengths larger than `MAX_SAFE_INTEGER` as array-like', function() {
+    assert.strictEqual(size({ 'length': MAX_SAFE_INTEGER + 1 }), 1);
+  });
+
+  it('should not treat objects with non-number lengths as array-like', function() {
+    assert.strictEqual(size({ 'length': '0' }), 1);
+  });
+});