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/findLastIndex-and-lastIndexOf.js

@@ -0,0 +1,65 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _, identity, stubZero, falsey } from './utils.js';
+
+describe('findLastIndex and lastIndexOf', function() {
+  lodashStable.each(['findLastIndex', 'lastIndexOf'], function(methodName) {
+    var array = [1, 2, 3, 1, 2, 3],
+        func = _[methodName],
+        resolve = methodName == 'findLastIndex' ? lodashStable.curry(lodashStable.eq) : identity;
+
+    it('`_.' + methodName + '` should return the index of the last matched value', function() {
+      assert.strictEqual(func(array, resolve(3)), 5);
+    });
+
+    it('`_.' + methodName + '` should work with a positive `fromIndex`', function() {
+      assert.strictEqual(func(array, resolve(1), 2), 0);
+    });
+
+    it('`_.' + methodName + '` should work with a `fromIndex` >= `length`', function() {
+      var values = [6, 8, Math.pow(2, 32), Infinity],
+          expected = lodashStable.map(values, lodashStable.constant([-1, 3, -1]));
+
+      var actual = lodashStable.map(values, function(fromIndex) {
+        return [
+          func(array, resolve(undefined), fromIndex),
+          func(array, resolve(1), fromIndex),
+          func(array, resolve(''), fromIndex)
+        ];
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    });
+
+    it('`_.' + methodName + '` should work with a negative `fromIndex`', function() {
+      assert.strictEqual(func(array, resolve(2), -3), 1);
+    });
+
+    it('`_.' + methodName + '` should work with a negative `fromIndex` <= `-length`', function() {
+      var values = [-6, -8, -Infinity],
+          expected = lodashStable.map(values, stubZero);
+
+      var actual = lodashStable.map(values, function(fromIndex) {
+        return func(array, resolve(1), fromIndex);
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    });
+
+    it('`_.' + methodName + '` should treat falsey `fromIndex` values correctly', function() {
+      var expected = lodashStable.map(falsey, function(value) {
+        return value === undefined ? 5 : -1;
+      });
+
+      var actual = lodashStable.map(falsey, function(fromIndex) {
+        return func(array, resolve(3), fromIndex);
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    });
+
+    it('`_.' + methodName + '` should coerce `fromIndex` to an integer', function() {
+      assert.strictEqual(func(array, resolve(2), 4.2), 4);
+    });
+  });
+});