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

Ashutosh Guptarequested to merge
gitlab-community/gitlab-org/gitlab:issue_405322 into master

This MR aims to solve #405322 (closed)

I have replaced references to loadHTMLFixture in the following places:

spec/frontend/issues/issue_spec.js:43: loadHTMLFixture('issues/open-issue.html');

spec/frontend/issues/issue_spec.js:45: loadHTMLFixture('issues/closed-issue.html');

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