[documentation] Fix link to junit5 extensions in testing.md

[Calfur]

Currently the link Extensions on https://insert-koin.io/docs/reference/koin-test/testing/#testing-with-junit5 redirects to the not existing url https://insert-koin.io/docs/reference/koin-test/testing/[url](https://junit.org/junit5/docs/current/user-guide/#extensions)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.