Review and improve tests

[mwcz]

Many elements have bare minimum testing. Review each high-priority component and make sure they all have at least one test of substance. More than one for elements with sufficient complexity.

High-priority components:

• rh-card
• rh-icon
• rh-icon-panel
• rh-button
• rh-cta

(rh-tabs is also priority but is already well tested)

[mwcz]

I also added a test to every element and to the generator that verifies the element was successfully upgraded.