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
```
You mean it does not cache if the build fails?