fix(accordion): update tests correct functionality

[zeroedin]

What I did

1. Expanded upon spec tests
2. Corrected expanded and expanded-index attributes functionality
3. Improved demo naming and added comment to header usage in default demo

Todo

• Add changeset, determine what level of change this is...
  Does this actually change the expected API? Currently if you add expanded to a header it does not unless on firstUpdated update the accordions state. I felt like that was incorrect expectation and that the panel should belong wot the header and expand with it when the header is given expanded. However even PFE accordion you need to add expanded to both the header and the panel in order for the pair to be expanded. Also you can now use expanded-index and expanded together now as both just feed the expandedIndexSet.

Closes #2032
Closes #2019

Testing Instructions

1. View Deploy preview demos
   Expanded Index Demo
   Expanded Attribute Demo

Notes to Reviewers

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[github-actions]

Size Change: +160 B (+0.08%)

Total Size: 207 kB

Filename	Size	Change
./elements/rh-accordion/rh-accordion-header.js	2.89 kB	+120 B (+4.34%)
./elements/rh-accordion/rh-accordion-panel.js	1.42 kB	+48 B (+3.51%)
./elements/rh-accordion/rh-accordion.js	3.2 kB	-8 B (-0.25%)

ℹ️ View Unchanged

Filename	Size
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_{compressed-size-action}

[zeroedin]

@bennypowers thoughts on the level of change here?