Robin Nater

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

1

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)