Fix dropdown filter tests

What does this MR do?

Fixes projects_dropdown_filter_spec and groups_dropdown_filter_spec test specs broken by a gitlab-ui upgrade.

Why were the tests broken by a gitlab-ui upgrade?

Because the components’ tests are referencing a class selector in the avatar component that does not exist anymore in gitlab-ui: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15659/diffs#15ae8d101788428e2fc2d4ff8da48c0e0f2f1f64_144_143

What is the solution?

This MR updates the selectors in the tests.

Closes #13819 (closed)