Unit tests in the gamnit module were not executed because Sys::setup exited right away when detecting NIT_TESTING. It affected all gamnit games. This PR check is a platform is bound at the beginning of `Sys::main`, allowing unit tests to run (whether they redef `main` or not).
The check is made possible by the new `bound_platform` which tells what platform is currently bound. This value can be useful in some other cases.
This is an alternative to #2394.
Pull-Request: #2420
Reviewed-by: Jean Privat <jean@pryen.org>