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/initial.js

@@ -0,0 +1,61 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { falsey, stubArray, LARGE_ARRAY_SIZE } from './utils.js';
+import initial from '../initial.js';
+
+describe('initial', function() {
+  var array = [1, 2, 3];
+
+  it('should accept a falsey `array`', function() {
+    var expected = lodashStable.map(falsey, stubArray);
+
+    var actual = lodashStable.map(falsey, function(array, index) {
+      try {
+        return index ? initial(array) : initial();
+      } catch (e) {}
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should exclude last element', function() {
+    assert.deepStrictEqual(initial(array), [1, 2]);
+  });
+
+  it('should return an empty when querying empty arrays', function() {
+    assert.deepStrictEqual(initial([]), []);
+  });
+
+  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, initial);
+
+    assert.deepStrictEqual(actual, [[1, 2], [4, 5], [7, 8]]);
+  });
+
+  it('should work in a lazy sequence', function() {
+    var array = lodashStable.range(LARGE_ARRAY_SIZE),
+        values = [];
+
+    var actual = _(array).initial().filter(function(value) {
+      values.push(value);
+      return false;
+    })
+    .value();
+
+    assert.deepEqual(actual, []);
+    assert.deepEqual(values, initial(array));
+
+    values = [];
+
+    actual = _(array).filter(function(value) {
+      values.push(value);
+      return isEven(value);
+    })
+    .initial()
+    .value();
+
+    assert.deepEqual(actual, initial(lodashStable.filter(array, isEven)));
+    assert.deepEqual(values, array);
+  });
+});