gitlab-ui tooltip component is based on bootstrap-vue tooltip
component. Starting with bootstrap-vue 2.0, testing Vue components
that use BV tooltip requires to attach the component under test to the
document object using vue-test-utils attachToDocument mounting option.
It also requires setting the `sync` option to true.

This change affected a significant amount of unit tests in GitLab
codebase. These effects impede GitLab from upgrading to
bootstrap-vue 2.0. To address this problem we first attempted to fix
all the affected tests. The list is broken tests is located in
https://gitlab.com/gitlab-org/gitlab/merge_requests/18913.

Over time, more tests appeared broken as they were using the tooltip
directive too. The goal to fix broken tests became a moving target,
so we desisted. This new approach mocks the tooltip directive and
tooltip component to decouple the unit tests from the bootstrap-vue
dependency.