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/unary.js

@@ -0,0 +1,27 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { slice } from './utils.js';
+import unary from '../unary.js';
+
+describe('unary', function() {
+  function fn() {
+    return slice.call(arguments);
+  }
+
+  it('should cap the number of arguments provided to `func`', function() {
+    var actual = lodashStable.map(['6', '8', '10'], unary(parseInt));
+    assert.deepStrictEqual(actual, [6, 8, 10]);
+  });
+
+  it('should not force a minimum argument count', function() {
+    var capped = unary(fn);
+    assert.deepStrictEqual(capped(), []);
+  });
+
+  it('should use `this` binding of function', function() {
+    var capped = unary(function(a, b) { return this; }),
+        object = { 'capped': capped };
+
+    assert.strictEqual(object.capped(), object);
+  });
+});