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/ary.js

@@ -0,0 +1,91 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { slice, _ } from './utils.js';
+import ary from '../ary.js';
+import curry from '../curry.js';
+import rearg from '../rearg.js';
+
+describe('ary', function() {
+  function fn(a, b, c) {
+    return slice.call(arguments);
+  }
+
+  it('should cap the number of arguments provided to `func`', function() {
+    var actual = lodashStable.map(['6', '8', '10'], ary(parseInt, 1));
+    assert.deepStrictEqual(actual, [6, 8, 10]);
+
+    var capped = ary(fn, 2);
+    assert.deepStrictEqual(capped('a', 'b', 'c', 'd'), ['a', 'b']);
+  });
+
+  it('should use `func.length` if `n` is not given', function() {
+    var capped = ary(fn);
+    assert.deepStrictEqual(capped('a', 'b', 'c', 'd'), ['a', 'b', 'c']);
+  });
+
+  it('should treat a negative `n` as `0`', function() {
+    var capped = ary(fn, -1);
+
+    try {
+      var actual = capped('a');
+    } catch (e) {}
+
+    assert.deepStrictEqual(actual, []);
+  });
+
+  it('should coerce `n` to an integer', function() {
+    var values = ['1', 1.6, 'xyz'],
+        expected = [['a'], ['a'], []];
+
+    var actual = lodashStable.map(values, function(n) {
+      var capped = ary(fn, n);
+      return capped('a', 'b');
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should not force a minimum argument count', function() {
+    var args = ['a', 'b', 'c'],
+        capped = ary(fn, 3);
+
+    var expected = lodashStable.map(args, function(arg, index) {
+      return args.slice(0, index);
+    });
+
+    var actual = lodashStable.map(expected, function(array) {
+      return capped.apply(undefined, array);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should use `this` binding of function', function() {
+    var capped = ary(function(a, b) { return this; }, 1),
+        object = { 'capped': capped };
+
+    assert.strictEqual(object.capped(), object);
+  });
+
+  it('should use the existing `ary` if smaller', function() {
+    var capped = ary(ary(fn, 1), 2);
+    assert.deepStrictEqual(capped('a', 'b', 'c'), ['a']);
+  });
+
+  it('should work as an iteratee for methods like `_.map`', function() {
+    var funcs = lodashStable.map([fn], ary),
+        actual = funcs[0]('a', 'b', 'c');
+
+    assert.deepStrictEqual(actual, ['a', 'b', 'c']);
+  });
+
+  it('should work when combined with other methods that use metadata', function() {
+    var array = ['a', 'b', 'c'],
+        includes = curry(rearg(ary(_.includes, 2), 1, 0), 2);
+
+    assert.strictEqual(includes('b')(array, 2), true);
+
+    includes = _(_.includes).ary(2).rearg(1, 0).curry(2).value();
+    assert.strictEqual(includes('b')(array, 2), true);
+  });
+});