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/isInteger-methods.js

@@ -0,0 +1,55 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _, stubTrue, MAX_INTEGER, stubFalse, falsey, args, symbol } from './utils.js';
+
+describe('isInteger methods', function() {
+  lodashStable.each(['isInteger', 'isSafeInteger'], function(methodName) {
+    var func = _[methodName],
+        isSafe = methodName == 'isSafeInteger';
+
+    it('`_.' + methodName + '` should return `true` for integer values', function() {
+      var values = [-1, 0, 1],
+          expected = lodashStable.map(values, stubTrue);
+
+      var actual = lodashStable.map(values, function(value) {
+        return func(value);
+      });
+
+      assert.deepStrictEqual(actual, expected);
+      assert.strictEqual(func(MAX_INTEGER), !isSafe);
+    });
+
+    it('should return `false` for non-integer number values', function() {
+      var values = [NaN, Infinity, -Infinity, Object(1), 3.14],
+          expected = lodashStable.map(values, stubFalse);
+
+      var actual = lodashStable.map(values, function(value) {
+        return func(value);
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    });
+
+    it('should return `false` for non-numeric values', function() {
+      var expected = lodashStable.map(falsey, function(value) {
+        return value === 0;
+      });
+
+      var actual = lodashStable.map(falsey, function(value, index) {
+        return index ? func(value) : func();
+      });
+
+      assert.deepStrictEqual(actual, expected);
+
+      assert.strictEqual(func(args), false);
+      assert.strictEqual(func([1, 2, 3]), false);
+      assert.strictEqual(func(true), false);
+      assert.strictEqual(func(new Date), false);
+      assert.strictEqual(func(new Error), false);
+      assert.strictEqual(func({ 'a': 1 }), false);
+      assert.strictEqual(func(/x/), false);
+      assert.strictEqual(func('a'), false);
+      assert.strictEqual(func(symbol), false);
+    });
+  });
+});