Refactor TestReports specific table css

Summary

As part of !18255 (merged), some specific table css rules were added which could have been generalised for better effect. Specifically, it was the hover effect applied to clickable rows in the table.

Improvements

Move these hover rules into the general responsive_tables.scss file so that these are available throughout and consistent.

Risks

• Minimal risks as these rules are only being used in one place right now.

Involved components

• pipelines > components > test_reports > test_summary_table.vue