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/repeat.js

@@ -0,0 +1,46 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { falsey, stubThree } from './utils.js';
+import repeat from '../repeat.js';
+
+describe('repeat', function() {
+  var string = 'abc';
+
+  it('should repeat a string `n` times', function() {
+    assert.strictEqual(repeat('*', 3), '***');
+    assert.strictEqual(repeat(string, 2), 'abcabc');
+  });
+
+  it('should treat falsey `n` values, except `undefined`, as `0`', function() {
+    var expected = lodashStable.map(falsey, function(value) {
+      return value === undefined ? string : '';
+    });
+
+    var actual = lodashStable.map(falsey, function(n, index) {
+      return index ? repeat(string, n) : repeat(string);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should return an empty string if `n` is <= `0`', function() {
+    assert.strictEqual(repeat(string, 0), '');
+    assert.strictEqual(repeat(string, -2), '');
+  });
+
+  it('should coerce `n` to an integer', function() {
+    assert.strictEqual(repeat(string, '2'), 'abcabc');
+    assert.strictEqual(repeat(string, 2.6), 'abcabc');
+    assert.strictEqual(repeat('*', { 'valueOf': stubThree }), '***');
+  });
+
+  it('should coerce `string` to a string', function() {
+    assert.strictEqual(repeat(Object(string), 2), 'abcabc');
+    assert.strictEqual(repeat({ 'toString': lodashStable.constant('*') }, 3), '***');
+  });
+
+  it('should work as an iteratee for methods like `_.map`', function() {
+    var actual = lodashStable.map(['a', 'b', 'c'], repeat);
+    assert.deepStrictEqual(actual, ['a', 'b', 'c']);
+  });
+});