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/isType-checks.js

@@ -0,0 +1,39 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { objToString, objectTag, _, xml } from './utils.js';
+
+describe('isType checks', function() {
+  it('should return `false` for subclassed values', function() {
+    var funcs = [
+      'isArray', 'isBoolean', 'isDate', 'isFunction',
+      'isNumber', 'isRegExp', 'isString'
+    ];
+
+    lodashStable.each(funcs, function(methodName) {
+      function Foo() {}
+      Foo.prototype = root[methodName.slice(2)].prototype;
+
+      var object = new Foo;
+      if (objToString.call(object) == objectTag) {
+        assert.strictEqual(_[methodName](object), false, '`_.' + methodName + '` returns `false`');
+      }
+    });
+  });
+
+  it('should not error on host objects (test in IE)', function() {
+    var funcs = [
+      'isArguments', 'isArray', 'isArrayBuffer', 'isArrayLike', 'isBoolean',
+      'isBuffer', 'isDate', 'isElement', 'isError', 'isFinite', 'isFunction',
+      'isInteger', 'isMap', 'isNaN', 'isNil', 'isNull', 'isNumber', 'isObject',
+      'isObjectLike', 'isRegExp', 'isSet', 'isSafeInteger', 'isString',
+      'isUndefined', 'isWeakMap', 'isWeakSet'
+    ];
+
+    lodashStable.each(funcs, function(methodName) {
+      if (xml) {
+        _[methodName](xml);
+        assert.ok(true, '`_.' + methodName + '` should not error');
+      }
+    });
+  });
+});