Add tc-test-case class to testcase-widget

[pmario]

This PR is intended for v5.3.7 This PR adds:

• tc-test-case default class to the testcase-widget
• New optional class parameter added to the same DOM element as default class
• Adjust documentaion

This is the first step to fix issue:

• 8741

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[Jermolene]

Thanks @pmario looks good for post-v5.3.6. I've made a note in #8745