Improve performance of end-to-end tests

What does this MR do?

Improve the performance of end-to-end tests by fabricating the issue resources via API call instead of via the GUI.

Note: This change also makes the tests more reliable since they depend less on the GUI.

It also removes duplicated strings from tests by storing them into variables and then these variables are reused.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content