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/split.js

@@ -0,0 +1,35 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import split from '../split.js';
+
+describe('split', function() {
+  it('should split a string by `separator`', function() {
+    var string = 'abcde';
+    assert.deepStrictEqual(split(string, 'c'), ['ab', 'de']);
+    assert.deepStrictEqual(split(string, /[bd]/), ['a', 'c', 'e']);
+    assert.deepStrictEqual(split(string, '', 2), ['a', 'b']);
+  });
+
+  it('should return an array containing an empty string for empty values', function() {
+    var values = [, null, undefined, ''],
+        expected = lodashStable.map(values, lodashStable.constant(['']));
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? split(value) : split();
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should work as an iteratee for methods like `_.map`', function() {
+    var strings = ['abc', 'def', 'ghi'],
+        actual = lodashStable.map(strings, split);
+
+    assert.deepStrictEqual(actual, [['abc'], ['def'], ['ghi']]);
+  });
+
+  it('should allow mixed string and array prototype methods', function() {
+    var wrapped = _('abc');
+    assert.strictEqual(wrapped.split('b').join(','), 'a,c');
+  });
+});