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/uniqueId.js

@@ -0,0 +1,22 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import uniqueId from '../uniqueId.js';
+
+describe('uniqueId', function() {
+  it('should generate unique ids', function() {
+    var actual = lodashStable.times(1000, function() {
+      return uniqueId();
+    });
+
+    assert.strictEqual(lodashStable.uniq(actual).length, actual.length);
+  });
+
+  it('should return a string value when not providing a `prefix`', function() {
+    assert.strictEqual(typeof uniqueId(), 'string');
+  });
+
+  it('should coerce the prefix argument to a string', function() {
+    var actual = [uniqueId(3), uniqueId(2), uniqueId(1)];
+    assert.ok(/3\d+,2\d+,1\d+/.test(actual));
+  });
+});