Update testing.md to reflect that we don't use yarn anymore

We don't use yarn since #45167

contributing/core/testing.md

@@ -38,8 +38,6 @@ When you run an e2e test, a local version of next will be created inside your sy
 which is then linked to the app, also created inside a temp folder. A server is started on a random port, against which the tests will run.
 After all tests have finished, the server is destroyed and all remaining files are deleted from the temp folder.
 
-You will need `yarn` for running e2e tests. Installing it `corepack` won't work because `next.js` is `pnpm` workspace.
-
 ## Writing tests for Next.js
 
 ### Getting Started