Remove computed props checks for MR widget specs

What does this MR do?

Remove computed props checks for MR widget specs, see #227708 (closed)

This change removes references to computed props on mr_widget_pipeline_spec.js (CE & EE), in favor of checking for rendered output or props passed to other components.

Additionally switches some usage of mount in favor of shallowMount.

Screenshots

N/A

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

Closes #227708 (closed)