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/dropRightWhile.js

@@ -0,0 +1,52 @@
+import assert from 'assert';
+import { slice } from './utils.js';
+import dropRightWhile from '../dropRightWhile.js';
+
+describe('dropRightWhile', function() {
+  var array = [1, 2, 3, 4];
+
+  var objects = [
+    { 'a': 0, 'b': 0 },
+    { 'a': 1, 'b': 1 },
+    { 'a': 2, 'b': 2 }
+  ];
+
+  it('should drop elements while `predicate` returns truthy', function() {
+    var actual = dropRightWhile(array, function(n) {
+      return n > 2;
+    });
+
+    assert.deepStrictEqual(actual, [1, 2]);
+  });
+
+  it('should provide correct `predicate` arguments', function() {
+    var args;
+
+    dropRightWhile(array, function() {
+      args = slice.call(arguments);
+    });
+
+    assert.deepStrictEqual(args, [4, 3, array]);
+  });
+
+  it('should work with `_.matches` shorthands', function() {
+    assert.deepStrictEqual(dropRightWhile(objects, { 'b': 2 }), objects.slice(0, 2));
+  });
+
+  it('should work with `_.matchesProperty` shorthands', function() {
+    assert.deepStrictEqual(dropRightWhile(objects, ['b', 2]), objects.slice(0, 2));
+  });
+
+  it('should work with `_.property` shorthands', function() {
+    assert.deepStrictEqual(dropRightWhile(objects, 'b'), objects.slice(0, 1));
+  });
+
+  it('should return a wrapped value when chaining', function() {
+    var wrapped = _(array).dropRightWhile(function(n) {
+      return n > 2;
+    });
+
+    assert.ok(wrapped instanceof _);
+    assert.deepEqual(wrapped.value(), [1, 2]);
+  });
+});