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/unionBy.js

@@ -0,0 +1,28 @@
+import assert from 'assert';
+import { slice } from './utils.js';
+import unionBy from '../unionBy.js';
+
+describe('unionBy', function() {
+  it('should accept an `iteratee`', function() {
+    var actual = unionBy([2.1], [1.2, 2.3], Math.floor);
+    assert.deepStrictEqual(actual, [2.1, 1.2]);
+
+    actual = unionBy([{ 'x': 1 }], [{ 'x': 2 }, { 'x': 1 }], 'x');
+    assert.deepStrictEqual(actual, [{ 'x': 1 }, { 'x': 2 }]);
+  });
+
+  it('should provide correct `iteratee` arguments', function() {
+    var args;
+
+    unionBy([2.1], [1.2, 2.3], function() {
+      args || (args = slice.call(arguments));
+    });
+
+    assert.deepStrictEqual(args, [2.1]);
+  });
+
+  it('should output values from the first possible array', function() {
+    var actual = unionBy([{ 'x': 1, 'y': 1 }], [{ 'x': 1, 'y': 2 }], 'x');
+    assert.deepStrictEqual(actual, [{ 'x': 1, 'y': 1 }]);
+  });
+});