Use non-existing example in tests (#20021)

This PR makes a small change to the tests. In some cases, where we test
`@utility` functionality we use `tab-*` utilities.

But there is a possibility that we will add this as an actual utility,
so instead let's use a much more generic `example-*` utility. Then we
can read from the `--example` theme value, and use `--resolved-value:
…;` and `--resolved-modifier: …;` values.

It's admittedly more vague, but chances of conflicts go way down. Until
CSS or Tailwind CSS introduces an `example-*` utility...


## Test plan

1. Only tests changed, and all of them still pass