DELETEME: test if Precompiled newlib is downloaded

This should not happen:

```
make -C ./tests/unittests clean all BOARD=qemu-i386
make: Entering directory '/home/ludwig/RIOT/tests/unittests'
..../RIOT/boards/x86-multiboot-common/Makefile.include:9: Precompiled newlib is missing in /home/ludwig/RIOT/toolchain/x86/i586-none-elf
..../RIOT/boards/x86-multiboot-common/Makefile.include:10: Downloading from http://download.riot-os.org/i586-newlib_2.1.0_tlsf-1254.tar.bz2
```
> I thought we already know what we wanted to know via: https://travis-ci.org/Kijewski/RIOT/builds/35939731#L944

Right, somehow I missed the part that it already should be cached. That's kinda the crucial part of the whole caching. :-/

I've set my master to this branch, maybe it changes something.