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

This MR aims to solve #405324 (closed)

I have replaced references to loadHTMLFixture in the following places:

spec/frontend/pages/admin/abuse_reports/abuse_reports_spec.js:18: loadHTMLFixture(FIXTURE);

spec/frontend/pages/admin/application_settings/account_and_limits_spec.js:11: loadHTMLFixture(FIXTURE);

spec/frontend/pages/admin/application_settings/metrics_and_profiling/usage_statistics_spec.js:15: loadHTMLFixture(FIXTURE);

spec/frontend/pages/dashboard/todos/index/todos_spec.js:21: loadHTMLFixture('todos/todos.html');

spec/frontend/pages/sessions/new/preserve_url_fragment_spec.js:11: loadHTMLFixture('sessions/new.html');

spec/frontend/pages/sessions/new/signin_tabs_memoizer_spec.js:23: loadHTMLFixture(fixtureTemplate);