Refactor Error Tracking Spec to Use Matchers

What does this MR do?

Refactor to use matchers instead of custom code.

I also updated to use let_it_be instead of set based on this guideline:

https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#set-variables

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers