Make diff specs compatible w/ BootstrapVue 2

What does this MR do?

This MR lays the groundwork for the BootstrapVue upgrade that will be done in this MR: !18913 (merged).

Due to some changes in the BTooltip and BPopover components, some related tests started failing after the upgrade. Those tests need to be updated accordingly:

• Migrate specs to use vue-test-utils
• attachToDocument must be true

This MR focuses on upgrading the tests located in:

• spec/frontend/diffs/components/diff_file_header_spec.js
• spec/frontend/diffs/components/diff_gutter_avatars_spec.js
• spec/frontend/diffs/components/edit_button_spec.js

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content