Run upstream testsuite

[sylvestre]

to verify that it works

We have this for coreutils:
https://github.com/uutils/coreutils/blob/main/util/run-gnu-test.sh
https://github.com/uutils/coreutils/blob/main/util/build-gnu.sh

or diffutils:
https://github.com/uutils/diffutils/tree/main/tests
see uutils/diffutils#8