chore(codequality): exclude test files from Code Quality

What does this MR do?

Excludes test files from Code Quality.

Closes #1945 (closed)

Changes in the analysis

477 issues are exlcuded.

Before

Found 849 code quality issues

https://gitlab.com/gitlab-org/gitlab-ui/-/pipelines/611496436/codequality_report

After

Found 372 code quality issues

https://gitlab.com/gitlab-org/gitlab-ui/-/pipelines/612180397/codequality_report

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [n/a] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [n/a] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [n/a] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • [n/a] GitLab: mr_url
  • [n/a] CustomersDot: mr_url
  • [n/a] Status Page: mr_url
• [n/a] Added the ~"component:*" label(s) if applicable.

Security

n/a

Accessibility

n/a