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/tap.js

@@ -0,0 +1,30 @@
+import assert from 'assert';
+
+describe('tap', function() {
+  it('should intercept and return the given value', function() {
+    var intercepted,
+        array = [1, 2, 3];
+
+    var actual = _.tap(array, function(value) {
+      intercepted = value;
+    });
+
+    assert.strictEqual(actual, array);
+    assert.strictEqual(intercepted, array);
+  });
+
+  it('should intercept unwrapped values and return wrapped values when chaining', function() {
+    var intercepted,
+        array = [1, 2, 3];
+
+    var wrapped = _(array).tap(function(value) {
+      intercepted = value;
+      value.pop();
+    });
+
+    assert.ok(wrapped instanceof _);
+
+    wrapped.value();
+    assert.strictEqual(intercepted, array);
+  });
+});