tests: Add run-tests-exp.py, simple MicroPython-based test runner.

This script is rewrite of run-tests-exp.sh, and tries to achieve self-hosted
testsuite running in environments where neither CPython nor unix shell is
available. As run-tests-exp.sh, it requires complete set of .exp files
pre-generated with ./run-test --write-exp.
It's just one file (".tst.out"). No, I don't want to rely on os.unlink() being builtin, and then it's system-specific "del" vs "rm" command. Users who will use this script should accept the need for manual cleanup ;-).