Move test scripts into a separate repo

[arsv]

The stuff from hacking_files is not going upsteam anyway, tests like that are not a part of common qemu codebase, but it's very important to have them organized and easily available for anyone willing to do any work on riscv-qemu.

And I think the new repo would become much more useful with reasonably generic build scripts. So that for instance one could pull the repo and run "make; make test" with only riscv64-unknown-elf toolchain and qemu-system-riscv64 in path, and with no other assumptions. This would naturally imply no hard-coded paths in the scripts.

The repo should probably look like riscv-tools, riscv-gnu-toolchain, i.e. build scripts + submodule for riscv-tests, and possibly riscv-linux once it gets that far.