regrtest: Synchronize regrtest in 3.11 and 3.12 branches with main branch

[vstinner]

Last weeks, it wasn't possible to synchronize regrtest from main to 3.12 since the 3.12 branch was blocked to prepare the final release. Now, there are tons of changes in main which were not backported to 3.12.

We can attempt to backport every single regrtest changes from main to 3.12, but it's a huge main to manage cherry-pick and all merge conflict. It did it once and it was really tricky and it's a fragile way.

I propose the trivial and reliable way: copy all files from main to 3.12 and 3.11 branches. It's reliable since we make sure that the code is consistent, no risk of missing a change or messing a resolution of a merge conflict.

Linked PRs

• [3.12] gh-110756: Sync regrtest with main branch #110758
• [3.12] gh-110756: Sync regrtest with main branch #110779
• [3.11] gh-110756: Sync regrtest with main branch (#110758) #110781
• [3.11] gh-110756: Fix libregrtest clear_caches() for distutils #111011