Replay the last N git commits on a different branch

Rebase should do it.

git rebase -p --onto master testing~10 testing

This will copy the last ten commits on testing to master and make that the new testing (the old testing will be an orphan). Then you can merge master to testing as a fast-forward.

git checkout master
git merge testing

1. git checkout master
2. git whatchanged testing
3. git cherry-pick _________

?

As said in comments, the rebase-inspired answer is leaving the 'garbage' commits orphaned.

Just use simple tools:

git checkout master
git merge testing
git checkout testing
git reset --hard HEAD~10   # Go back 10 commits (*1)
git checkout master

(*1) You will only be "losing" commits from the testing branch, since you'll have those commits in master thanks to the merge.