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/nth.js

@@ -0,0 +1,69 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { falsey, stubA, stubB, noop } from './utils.js';
+import nth from '../nth.js';
+
+describe('nth', function() {
+  var array = ['a', 'b', 'c', 'd'];
+
+  it('should get the nth element of `array`', function() {
+    var actual = lodashStable.map(array, function(value, index) {
+      return nth(array, index);
+    });
+
+    assert.deepStrictEqual(actual, array);
+  });
+
+  it('should work with a negative `n`', function() {
+    var actual = lodashStable.map(lodashStable.range(1, array.length + 1), function(n) {
+      return nth(array, -n);
+    });
+
+    assert.deepStrictEqual(actual, ['d', 'c', 'b', 'a']);
+  });
+
+  it('should coerce `n` to an integer', function() {
+    var values = falsey,
+        expected = lodashStable.map(values, stubA);
+
+    var actual = lodashStable.map(values, function(n) {
+      return n ? nth(array, n) : nth(array);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+
+    values = ['1', 1.6];
+    expected = lodashStable.map(values, stubB);
+
+    actual = lodashStable.map(values, function(n) {
+      return nth(array, n);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should return `undefined` for empty arrays', function() {
+    var values = [null, undefined, []],
+        expected = lodashStable.map(values, noop);
+
+    var actual = lodashStable.map(values, function(array) {
+      return nth(array, 1);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should return `undefined` for non-indexes', function() {
+    var array = [1, 2],
+        values = [Infinity, array.length],
+        expected = lodashStable.map(values, noop);
+
+    array[-1] = 3;
+
+    var actual = lodashStable.map(values, function(n) {
+      return nth(array, n);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+});