Change `sign_out` usage to `gitlab_sign_out` in one spec

For reasons that are still unclear, the EE version (but only the EE
version) of this spec was failing reliably, and changing these calls
seemed to resolve it. So we're making the change here for parity.

spec/features/issues/user_uses_slash_commands_spec.rb

@@ -41,7 +41,7 @@ feature 'Issues > User uses quick actions', feature: true, js: true do
         let(:guest) { create(:user) }
         before do
           project.team << [guest, :guest]
-          sign_out(:user)
+          gitlab_sign_out
           sign_in(guest)
           visit project_issue_path(project, issue)
         end
@@ -81,7 +81,7 @@ feature 'Issues > User uses quick actions', feature: true, js: true do
         let(:guest) { create(:user) }
         before do
           project.team << [guest, :guest]
-          sign_out(:user)
+          gitlab_sign_out
           sign_in(guest)
           visit project_issue_path(project, issue)
         end