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/before.js

@@ -0,0 +1,31 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _ } from './utils.js';
+
+describe('before', function() {
+  function before(n, times) {
+    var count = 0;
+    lodashStable.times(times, _.before(n, function() { count++; }));
+    return count;
+  }
+
+  it('should create a function that invokes `func` after `n` calls', function() {
+    assert.strictEqual(before(5, 4), 4, 'before(n) should invoke `func` before being called `n` times');
+    assert.strictEqual(before(5, 6), 4, 'before(n) should not invoke `func` after being called `n - 1` times');
+    assert.strictEqual(before(0, 0), 0, 'before(0) should not invoke `func` immediately');
+    assert.strictEqual(before(0, 1), 0, 'before(0) should not invoke `func` when called');
+  });
+
+  it('should coerce `n` values of `NaN` to `0`', function() {
+    assert.strictEqual(before(NaN, 1), 0);
+  });
+
+  it('should use `this` binding of function', function() {
+    var before = _.before(2, function() { return ++this.count; }),
+        object = { 'before': before, 'count': 0 };
+
+    object.before();
+    assert.strictEqual(object.before(), 1);
+    assert.strictEqual(object.count, 1);
+  });
+});