Add tests for attributes

Add rail click test.

fixes #2531569. Slider thumb not focasable by click

Better circular op protection for set(value, x) Fixes #2531498 The protection against `slider.set('value', x)` looping back from the thumb position was inadequate when setting the value by increments that would be rounded out when applying the value=>offset calculation. The lack of new pixel position resulted in the value being reverted. This was especially harmful to keyboard interaction on Sliders where the `max - min` was less than the pixel length, because arrow keys would attempt to move the value by 1, but would be stopped, thus preventing the thumb from moving further. Answer: respect the assigned value, duh.

Adding failing Slider test to _should.fail The bug (2531498) is still outstanding. Also updated the tests/src file FWIW.

fixes #2528788. ARIA labelledby change. Added unit tests keyboard and ARIA

fixes #2528788. Adding unit tests for keyboard input and ARIA

[fix #2530561] Cleanup slider css -> slider-base. Update failing tests. Update user guide. Update apidocs.

Move tests from sandbox to src/X/tests/src Adapt to test structure enforced by new builder target 'tests'. Still more cleanup to do. But hopefully I didn't miss anything. C'mon CI!