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/isMap.test.js

@@ -0,0 +1,53 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { map, falsey, stubFalse, args, slice, symbol, weakMap, realm } from './utils.js';
+import isMap from '../isMap.js';
+
+describe('isMap', function() {
+  it('should return `true` for maps', function() {
+    if (Map) {
+      assert.strictEqual(isMap(map), true);
+    }
+  });
+
+  it('should return `false` for non-maps', function() {
+    var expected = lodashStable.map(falsey, stubFalse);
+
+    var actual = lodashStable.map(falsey, function(value, index) {
+      return index ? isMap(value) : isMap();
+    });
+
+    assert.deepStrictEqual(actual, expected);
+
+    assert.strictEqual(isMap(args), false);
+    assert.strictEqual(isMap([1, 2, 3]), false);
+    assert.strictEqual(isMap(true), false);
+    assert.strictEqual(isMap(new Date), false);
+    assert.strictEqual(isMap(new Error), false);
+    assert.strictEqual(isMap(_), false);
+    assert.strictEqual(isMap(slice), false);
+    assert.strictEqual(isMap({ 'a': 1 }), false);
+    assert.strictEqual(isMap(1), false);
+    assert.strictEqual(isMap(/x/), false);
+    assert.strictEqual(isMap('a'), false);
+    assert.strictEqual(isMap(symbol), false);
+    assert.strictEqual(isMap(weakMap), false);
+  });
+
+  it('should work for objects with a non-function `constructor` (test in IE 11)', function() {
+    var values = [false, true],
+        expected = lodashStable.map(values, stubFalse);
+
+    var actual = lodashStable.map(values, function(value) {
+      return isMap({ 'constructor': value });
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should work with maps from another realm', function() {
+    if (realm.map) {
+      assert.strictEqual(isMap(realm.map), true);
+    }
+  });
+});