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/escapeRegExp.js

@@ -0,0 +1,28 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { stubString } from './utils.js';
+import escapeRegExp from '../escapeRegExp.js';
+
+describe('escapeRegExp', function() {
+  var escaped = '\\^\\$\\.\\*\\+\\?\\(\\)\\[\\]\\{\\}\\|\\\\',
+      unescaped = '^$.*+?()[]{}|\\';
+
+  it('should escape values', function() {
+    assert.strictEqual(escapeRegExp(unescaped + unescaped), escaped + escaped);
+  });
+
+  it('should handle strings with nothing to escape', function() {
+    assert.strictEqual(escapeRegExp('abc'), 'abc');
+  });
+
+  it('should return an empty string for empty values', function() {
+    var values = [, null, undefined, ''],
+        expected = lodashStable.map(values, stubString);
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? escapeRegExp(value) : escapeRegExp();
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+});