Fix shallowMount + stubs behavior change in `@vue/test-utils`

What does this MR do?

This MR fixes shallowMount + mount changes incompatibilities in upcoming @vue/test-utils update

See #145615 (closed) for details

To workaround this, we're switching from shallowMount to mount + stubbing all child components, except specific ones. This allows us to fully render GlTable without knowing anything about its internal implementation details.

Closes #145615 (closed)

Does this MR meet the acceptance criteria?

Conformity

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