Change Harness::run to run until no more repaints are requested (#5580)

Previously, `Harness::run` just called `Harness::step` 3 times. If that
wasn't enough, tests would often call run multiple times so all
animations would finish properly.

Also, I introduced `HarnessBuilder::with_step_dt` to customize with how
big of a dt each frame is called. I set the default to 1.0 / 6.0 (~6fps)
so we don't waste cpu in tests waiting on animations.

`HarnessBuilder::max_steps` allows us to control how many steps
`Harness::run` should run before panicing.
The default is 6, so we run for up to 1.0 logical seconds (six frames at
6 fps), which should be enough to finish most animations.

Turns out a lot of snapshots where rendered before fully shown and had a
light opacity, those are now fixed.

* [x] I have followed the instructions in the PR template

crates/egui_demo_lib/tests/snapshots/demos/Scrolling.png

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