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/multiply.test.js

@@ -0,0 +1,15 @@
+import assert from 'assert';
+import multiply from '../multiply.js';
+
+describe('multiply', function() {
+  it('should multiply two numbers', function() {
+    assert.strictEqual(multiply(6, 4), 24);
+    assert.strictEqual(multiply(-6, 4), -24);
+    assert.strictEqual(multiply(-6, -4), 24);
+  });
+
+  it('should coerce arguments to numbers', function() {
+    assert.strictEqual(multiply('6', '4'), 24);
+    assert.deepStrictEqual(multiply('x', 'y'), NaN);
+  });
+});