Remove loadHTMLFixture usage in `spec/frontend/filtered_search/`

This MR aims to solve #405321 (closed)

I have replaced references to loadHTMLFixture in the following places:

spec/frontend/filtered_search/dropdown_user_spec.js:80: loadHTMLFixture(fixtureTemplate);

spec/frontend/filtered_search/dropdown_utils_spec.js:358: loadHTMLFixture(issuableListFixture);