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/toInteger-methods.js

@@ -0,0 +1,25 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _, MAX_SAFE_INTEGER, MAX_INTEGER } from './utils.js';
+
+describe('toInteger methods', function() {
+  lodashStable.each(['toInteger', 'toSafeInteger'], function(methodName) {
+    var func = _[methodName],
+        isSafe = methodName == 'toSafeInteger';
+
+    it('`_.' + methodName + '` should convert values to integers', function() {
+      assert.strictEqual(func(-5.6), -5);
+      assert.strictEqual(func('5.6'), 5);
+      assert.strictEqual(func(), 0);
+      assert.strictEqual(func(NaN), 0);
+
+      var expected = isSafe ? MAX_SAFE_INTEGER : MAX_INTEGER;
+      assert.strictEqual(func(Infinity), expected);
+      assert.strictEqual(func(-Infinity), -expected);
+    });
+
+    it('`_.' + methodName + '` should support `value` of `-0`', function() {
+      assert.strictEqual(1 / func(-0), -Infinity);
+    });
+  });
+});