fix(GlDatepicker): adds tests

mentioned in commit 483f3629

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1108 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!168482 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1109 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!352 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)