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/sortedIndexBy-methods.js

@@ -0,0 +1,52 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _, slice, MAX_ARRAY_LENGTH, MAX_ARRAY_INDEX } from './utils.js';
+
+describe('sortedIndexBy methods', function() {
+  lodashStable.each(['sortedIndexBy', 'sortedLastIndexBy'], function(methodName) {
+    var func = _[methodName],
+        isSortedIndexBy = methodName == 'sortedIndexBy';
+
+    it('`_.' + methodName + '` should provide correct `iteratee` arguments', function() {
+      var args;
+
+      func([30, 50], 40, function() {
+        args || (args = slice.call(arguments));
+      });
+
+      assert.deepStrictEqual(args, [40]);
+    });
+
+    it('`_.' + methodName + '` should work with `_.property` shorthands', function() {
+      var objects = [{ 'x': 30 }, { 'x': 50 }],
+          actual = func(objects, { 'x': 40 }, 'x');
+
+      assert.strictEqual(actual, 1);
+    });
+
+    it('`_.' + methodName + '` should support arrays larger than `MAX_ARRAY_LENGTH / 2`', function() {
+      lodashStable.each([Math.ceil(MAX_ARRAY_LENGTH / 2), MAX_ARRAY_LENGTH], function(length) {
+        var array = [],
+            values = [MAX_ARRAY_LENGTH, NaN, undefined];
+
+        array.length = length;
+
+        lodashStable.each(values, function(value) {
+          var steps = 0;
+
+          var actual = func(array, value, function(value) {
+            steps++;
+            return value;
+          });
+
+          var expected = (isSortedIndexBy ? !lodashStable.isNaN(value) : lodashStable.isFinite(value))
+            ? 0
+            : Math.min(length, MAX_ARRAY_INDEX);
+
+          assert.ok(steps == 32 || steps == 33);
+          assert.strictEqual(actual, expected);
+        });
+      });
+    });
+  });
+});