Merge branch 'add-element-extensions-tests' into 'master'

Added element extensions spec for .matches and .closest

## What does this MR do?

Adds tests for `Element.prototype.matches` and `Element.prototype.closest`.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !7755

spec/javascripts/extensions/element_spec.js.es6

+/*= require extensions/element */
+
+(() => {
+  describe('Element extensions', function () {
+    beforeEach(() => {
+      this.element = document.createElement('ul');
+    });
+
+    describe('matches', () => {
+      it('returns true if element matches the selector', () => {
+        expect(this.element.matches('ul')).toBeTruthy();
+      });
+
+      it("returns false if element doesn't match the selector", () => {
+        expect(this.element.matches('.not-an-element')).toBeFalsy();
+      });
+    });
+
+    describe('closest', () => {
+      beforeEach(() => {
+        this.childElement = document.createElement('li');
+        this.element.appendChild(this.childElement);
+      });
+
+      it('returns the closest parent that matches the selector', () => {
+        expect(this.childElement.closest('ul').toString()).toBe(this.element.toString());
+      });
+
+      it('returns itself if it matches the selector', () => {
+        expect(this.childElement.closest('li').toString()).toBe(this.childElement.toString());
+      });
+
+      it('returns undefined if nothing matches the selector', () => {
+        expect(this.childElement.closest('.no-an-element')).toBeFalsy();
+      });
+    });
+  });
+})();