Add all slots to GlPopover mock

What does this MR do?

This MR spreads all the slots in GlPopover mock to allow to test without manual mocks

Screenshots

No visual changes

Component definition

• GlPopover => https://gitlab.com/gitlab-org/gitlab-ui/-/blob/master/src/components/base/popover/popover.vue
• b-popover => https://bootstrap-vue.js.org/docs/components/popover

Open questions

Should we also spread all the scopedSlots for future compatibility? Is this something that is worth to do for all the components?

Closes #213194 (closed)