Add tests for date_picker behavior

spec/frontend/behaviors/date_picker_spec.js

+import * as Pikaday from 'pikaday';
+import initDatePickers from '~/behaviors/date_picker';
+import * as utils from '~/lib/utils/datetime_utility';
+
+jest.mock('pikaday');
+jest.mock('~/lib/utils/datetime_utility');
+
+describe('date_picker behavior', () => {
+  let pikadayMock;
+  let parseMock;
+
+  beforeEach(() => {
+    pikadayMock = jest.spyOn(Pikaday, 'default');
+    parseMock = jest.spyOn(utils, 'parsePikadayDate');
+    setFixtures(`
+      <div>
+        <input class="datepicker" value="2020-10-01" />
+      </div>
+      <div>
+        <input class="datepicker" value="" />
+      </div>`);
+  });
+
+  it('Instantiates Pickaday for every instance of a .datepicker class', () => {
+    initDatePickers();
+
+    expect(pikadayMock.mock.calls.length).toEqual(2);
+    expect(parseMock.mock.calls).toEqual([['2020-10-01'], ['']]);
+  });
+});