Integrate the busybox tests suite results

[sylvestre]

Just like we are doing with GNU, we should store and publish the results.

make prepare-busytest
        bindir=$(pwd)/target/debug
        cd tmp/busybox-*/testsuite
        output=$(bindir=$bindir ./runtest 2>&1 || true)
        printf "%s\n" "${output}"
        n_fails=$(echo "$output" | grep "^FAIL:\s" | wc --lines)
        if [ $n_fails -gt 0 ] ; then echo "::warning ::${n_fails}+ test failures" ; fi

is that is executed in the CI

[sylvestre]

GNU tests: https://github.com/uutils/coreutils/blob/main/.github/workflows/GnuTests.yml Used by https://github.com/uutils/coreutils-tracking