Fix existing_errors for AccessibilityReportsComparer

What does this MR do?

This MR fixes how we calculate our existing_errors method between 2 accessibility reports.

Instead of returning all existing errors from the base_report we are now getting the intersection between the base and the head report.

It also refactors our specs to clean some ~"technical debt".

Why are we doing this?

@shinya.maeda pointed out that we where doing the wrong calculation for our existing_errors when working on our CodequalityReportsComparer.

We discovered that this is also the case with our AccessibilityReportsComparer. This MR fixes this ~bug.

Does this MR meet the acceptance criteria?

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team