Use `assert.strictEqual`/`assert.true`/`assert.false`

This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.

element

* Remove aria-expanded from collapse.js target element

aria-expanded="true"/aria-expanded="false" only applies to the trigger,
not the element that is being expanded/collapsed.

* Tweak collapse.js accessibility section

...to make it clearer that the aria-expanded attribute always just goes
on the control.

* Fix collapse.js unit tests

- reword some of the text to make it clear we're checking behavior of
trigger/control
- move incorrect aria-expanded out of the <div>s and to the actual
trigger/control <a>s
- fix incorrect test assertion text output false -> true

Fix accordion behavior with JQuery interface

To ensure that we don't accidentally use any of the aliases.
This should prevent any future regressions from #12761.
Also updates the test suite since it now can't use these aliases either.

Appears to be unnecessary and is broken by #16011,
which is otherwise quite sane.

Thanks to @peterblazejewicz & @Nikita240
Adds unit tests based on #14417
X-Ref: #14282
Closes #15807

[Fixes #15953] Implement `assert.expect` in each unit test

Fixes #15741
Closes #15751 by merging it.

Refs #15893

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