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/deburr.test.js

@@ -0,0 +1,28 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { burredLetters, deburredLetters, comboMarks } from './utils.js';
+import deburr from '../deburr.js';
+
+describe('deburr', function() {
+  it('should convert Latin Unicode letters to basic Latin', function() {
+    var actual = lodashStable.map(burredLetters, deburr);
+    assert.deepStrictEqual(actual, deburredLetters);
+  });
+
+  it('should not deburr Latin mathematical operators', function() {
+    var operators = ['\xd7', '\xf7'],
+        actual = lodashStable.map(operators, deburr);
+
+    assert.deepStrictEqual(actual, operators);
+  });
+
+  it('should deburr combining diacritical marks', function() {
+    var expected = lodashStable.map(comboMarks, lodashStable.constant('ei'));
+
+    var actual = lodashStable.map(comboMarks, function(chr) {
+      return deburr('e' + chr + 'i');
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+});