Make feature flags specs compatible w/ BootstrapVue 2

Prepare feature flags specs for BootstrapVue upgrade

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
• sync needs to be set to false
• attachToDocument must be true

This MR focuses on upgrading the tests located in ee/spec/frontend/feature_flags/:

Why are these changes necessary?

• The reason to set attachDocument to true is that the BTooltip and BPopover directives expect to be attached to a document object.
• The reason to set sync to false is that otherwise, specs for components that use BTooltip or BPopover will fail with the following error:

TypeError: Cannot read property 'sync' of null

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers