Remove uses of `getChildInstances` in FE tests

Description

getChildInstances is a function that returns children of a Vue instance that are instances of a given Vue component. This effectively is what find(...) is in vue-test-utils. Unfortunately, this function randomly stops working when a component is touched by vue-test-utils.

This is related to the unknown karma issues causing some vue-test-utils weirdness https://gitlab.com/gitlab-org/gitlab-ce/issues/56317.

Proposal

We should remove this function and replace uses of it with the preferred vue-test-utils.

/cc @adrielsantiago