Add a tests directory where one can run official RISCV tests, std benchmarks, and any other relevant test application. This would be a first step towards a regression testing.

some interesting places to get tests and benchmarks:
https://github.com/riscv/riscv-tests/tree/master/benchmarks
https://github.com/2122aaor/c-class-clone/tree/master/verification/tests/directed/benchmarks/coremark
https://github.com/cliffordwolf/picorv32/tree/master/dhrystone
https://github.com/unrn-riscv-contest/unrn-riscv-compliance/tree/master/riscv-ovpsim/examples
https://github.com/rv8-io/rv8-bench
https://github.com/embench/embench-iot/tree/master/src
https://github.com/scarv/scarv-cpu/tree/master/flow/embench
https://github.com/sjohann81/hf-risc/tree/master/software/app