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.
Nope, files open in text mode on Windows should have "\n" as EOL.