tests/lua_loader: remove deprecated test target
tests/lua_loader: remove deprecated test target

The test target is now defined in the common `Makefile.include`.
This removes the warning

    Makefile: warning: overriding recipe for target 'test'
    Makefile.include: warning: ignoring old recipe for target 'test'

mtausig/RIOT,smlng/RIOT,OlegHahm/RIOT,aeneby/RIOT,toonst/RIOT,aeneby/RIOT,aeneby/RIOT,basilfx/RIOT,jasonatran/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,cladmi/RIOT,x3ro/RIOT,jasonatran/RIOT,OTAkeys/RIOT,yogo1212/RIOT,toonst/RIOT,OlegHahm/RIOT,josephnoir/RIOT,basilfx/RIOT,jasonatran/RIOT,mtausig/RIOT,kYc0o/RIOT,mtausig/RIOT,RIOT-OS/RIOT,ant9000/RIOT,basilfx/RIOT,ant9000/RIOT,authmillenon/RIOT,smlng/RIOT,rfuentess/RIOT,ant9000/RIOT,OlegHahm/RIOT,toonst/RIOT,cladmi/RIOT,RIOT-OS/RIOT,x3ro/RIOT,miri64/RIOT,toonst/RIOT,miri64/RIOT,aeneby/RIOT,josephnoir/RIOT,rfuentess/RIOT,aeneby/RIOT,OTAkeys/RIOT,smlng/RIOT,josephnoir/RIOT,x3ro/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,yogo1212/RIOT,cladmi/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,toonst/RIOT,jasonatran/RIOT,mtausig/RIOT,OlegHahm/RIOT,kaspar030/RIOT,josephnoir/RIOT,cladmi/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,x3ro/RIOT,authmillenon/RIOT,miri64/RIOT,smlng/RIOT,kYc0o/RIOT,OlegHahm/RIOT,rfuentess/RIOT,x3ro/RIOT,authmillenon/RIOT,rfuentess/RIOT,rfuentess/RIOT,smlng/RIOT,yogo1212/RIOT,yogo1212/RIOT,basilfx/RIOT,yogo1212/RIOT,cladmi/RIOT,OTAkeys/RIOT,kaspar030/RIOT,authmillenon/RIOT,mtausig/RIOT,yogo1212/RIOT,kaspar030/RIOT