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/head.js

@@ -0,0 +1,62 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { arrayProto, LARGE_ARRAY_SIZE } from './utils.js';
+import head from '../head.js';
+import first from '../first.js';
+
+describe('head', function() {
+  var array = [1, 2, 3, 4];
+
+  it('should return the first element', function() {
+    assert.strictEqual(head(array), 1);
+  });
+
+  it('should return `undefined` when querying empty arrays', function() {
+    arrayProto[0] = 1;
+    assert.strictEqual(head([]), undefined);
+    arrayProto.length = 0;
+  });
+
+  it('should work as an iteratee for methods like `_.map`', function() {
+    var array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]],
+        actual = lodashStable.map(array, head);
+
+    assert.deepStrictEqual(actual, [1, 4, 7]);
+  });
+
+  it('should be aliased', function() {
+    assert.strictEqual(first, head);
+  });
+
+  it('should return an unwrapped value when implicitly chaining', function() {
+    var wrapped = _(array);
+    assert.strictEqual(wrapped.head(), 1);
+    assert.strictEqual(wrapped.first(), 1);
+  });
+
+  it('should return a wrapped value when explicitly chaining', function() {
+    var wrapped = _(array).chain();
+    assert.ok(wrapped.head() instanceof _);
+    assert.ok(wrapped.first() instanceof _);
+  });
+
+  it('should not execute immediately when explicitly chaining', function() {
+    var wrapped = _(array).chain();
+    assert.strictEqual(wrapped.head().__wrapped__, array);
+    assert.strictEqual(wrapped.first().__wrapped__, array);
+  });
+
+  it('should work in a lazy sequence', function() {
+    var largeArray = lodashStable.range(LARGE_ARRAY_SIZE),
+        smallArray = array;
+
+    lodashStable.each(['head', 'first'], function(methodName) {
+      lodashStable.times(2, function(index) {
+        var array = index ? largeArray : smallArray,
+            actual = _(array).filter(isEven)[methodName]();
+
+        assert.strictEqual(actual, _[methodName](_.filter(array, isEven)));
+      });
+    });
+  });
+});