tests: buttons: Fix build failure if BTN0_PIN is not declared
tests: buttons: Fix build failure if BTN0_PIN is not declared

On some boards, button 0 does not exist so BTN0_PIN is not defined, but
these boards may define other buttons.

Signed-off-by: Francois Berder <880c5b95aa239388d35adb184ba52ce6e74e78eb@users.noreply.github.com>

kYc0o/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,basilfx/RIOT,OlegHahm/RIOT,ant9000/RIOT,basilfx/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,authmillenon/RIOT,OTAkeys/RIOT,basilfx/RIOT,yogo1212/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,yogo1212/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,miri64/RIOT,OTAkeys/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kYc0o/RIOT,kaspar030/RIOT,basilfx/RIOT,kYc0o/RIOT,jasonatran/RIOT,yogo1212/RIOT,kaspar030/RIOT,basilfx/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,yogo1212/RIOT,yogo1212/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT