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/indexOf-methods.js

@@ -0,0 +1,63 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _, falsey } from './utils.js';
+
+describe('indexOf methods', function() {
+  lodashStable.each(['indexOf', 'lastIndexOf', 'sortedIndexOf', 'sortedLastIndexOf'], function(methodName) {
+    var func = _[methodName],
+        isIndexOf = !/last/i.test(methodName),
+        isSorted = /^sorted/.test(methodName);
+
+    it('`_.' + methodName + '` should accept a falsey `array`', function() {
+      var expected = lodashStable.map(falsey, lodashStable.constant(-1));
+
+      var actual = lodashStable.map(falsey, function(array, index) {
+        try {
+          return index ? func(array) : func();
+        } catch (e) {}
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    });
+
+    it('`_.' + methodName + '` should return `-1` for an unmatched value', function() {
+      var array = [1, 2, 3],
+          empty = [];
+
+      assert.strictEqual(func(array, 4), -1);
+      assert.strictEqual(func(array, 4, true), -1);
+      assert.strictEqual(func(array, undefined, true), -1);
+
+      assert.strictEqual(func(empty, undefined), -1);
+      assert.strictEqual(func(empty, undefined, true), -1);
+    });
+
+    it('`_.' + methodName + '` should not match values on empty arrays', function() {
+      var array = [];
+      array[-1] = 0;
+
+      assert.strictEqual(func(array, undefined), -1);
+      assert.strictEqual(func(array, 0, true), -1);
+    });
+
+    it('`_.' + methodName + '` should match `NaN`', function() {
+      var array = isSorted
+        ? [1, 2, NaN, NaN]
+        : [1, NaN, 3, NaN, 5, NaN];
+
+      if (isSorted) {
+        assert.strictEqual(func(array, NaN, true), isIndexOf ? 2 : 3);
+      }
+      else {
+        assert.strictEqual(func(array, NaN), isIndexOf ? 1 : 5);
+        assert.strictEqual(func(array, NaN, 2), isIndexOf ? 3 : 1);
+        assert.strictEqual(func(array, NaN, -2), isIndexOf ? 5 : 3);
+      }
+    });
+
+    it('`_.' + methodName + '` should match `-0` as `0`', function() {
+      assert.strictEqual(func([-0], 0), 0);
+      assert.strictEqual(func([0], -0), 0);
+    });
+  });
+});