Rollup merge of #147903 - Zalathar:edition, r=jieyouxu

compiletest: Store the selected edition in `TestProps`

While working on a larger overhaul of directive processing, I ran into difficulty with the `has_edition` local variable.

Storing the selected edition in `TestProps` should make it easier to extract parts of directive processing into independent handler functions, because the `//@ edition` handler won't need access to additional mutable state outside of `TestProps`.

We still automatically add the edition to `compile_flags`, because there is too much existing compiletest code relying on that behaviour.

r? jieyouxu

Author: Zalathar