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/pad-methods.js

@@ -0,0 +1,51 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _ } from './utils.js';
+import pad from '../pad.js';
+
+describe('pad methods', function() {
+  lodashStable.each(['pad', 'padStart', 'padEnd'], function(methodName) {
+    var func = _[methodName],
+        isPad = methodName == 'pad',
+        isStart = methodName == 'padStart',
+        string = 'abc';
+
+    it('`_.' + methodName + '` should not pad if string is >= `length`', function() {
+      assert.strictEqual(func(string, 2), string);
+      assert.strictEqual(func(string, 3), string);
+    });
+
+    it('`_.' + methodName + '` should treat negative `length` as `0`', function() {
+      lodashStable.each([0, -2], function(length) {
+        assert.strictEqual(func(string, length), string);
+      });
+    });
+
+    it('`_.' + methodName + '` should coerce `length` to a number', function() {
+      lodashStable.each(['', '4'], function(length) {
+        var actual = length ? (isStart ? ' abc' : 'abc ') : string;
+        assert.strictEqual(func(string, length), actual);
+      });
+    });
+
+    it('`_.' + methodName + '` should treat nullish values as empty strings', function() {
+      lodashStable.each([undefined, '_-'], function(chars) {
+        var expected = chars ? (isPad ? '__' : chars) : '  ';
+        assert.strictEqual(func(null, 2, chars), expected);
+        assert.strictEqual(func(undefined, 2, chars), expected);
+        assert.strictEqual(func('', 2, chars), expected);
+      });
+    });
+
+    it('`_.' + methodName + '` should return `string` when `chars` coerces to an empty string', function() {
+      var values = ['', Object('')],
+          expected = lodashStable.map(values, lodashStable.constant(string));
+
+      var actual = lodashStable.map(values, function(value) {
+        return pad(string, 6, value);
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    });
+  });
+});