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

This MR aims to solve #405317 (closed)

I have replaced references to loadHTMLFixture in the following places:

spec/frontend/behaviors/quick_submit_spec.js:11: loadHTMLFixture('snippets/show.html');

spec/frontend/behaviors/requires_input_spec.js:9: loadHTMLFixture('branches/new_branch.html');

spec/frontend/behaviors/shortcuts/shortcuts_issuable_spec.js:27: loadHTMLFixture(snippetShowFixtureName);