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/add.test.js

@@ -0,0 +1,15 @@
+import assert from 'assert';
+import add from '../add.js';
+
+describe('add', function() {
+  it('should add two numbers', function() {
+    assert.strictEqual(add(6, 4), 10);
+    assert.strictEqual(add(-6, 4), -2);
+    assert.strictEqual(add(-6, -4), -10);
+  });
+
+  it('should not coerce arguments to numbers', function() {
+    assert.strictEqual(add('6', '4'), '64');
+    assert.strictEqual(add('x', 'y'), 'xy');
+  });
+});