tests : conditional python & node json schema tests (#6207)

* json: only attempt python & node schema conversion tests if their bins are present

Tests introduced in https://github.com/ggerganov/llama.cpp/pull/5978
disabled in https://github.com/ggerganov/llama.cpp/pull/6198

* json: orange warnings when tests skipped

* json: ensure py/js schema conv tested on ubuntu-focal-make

* json: print env vars in test