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.
Don't you need to strip `\r\n` on windows?  Then you can just use: `testcases = [l.strip() for l in f.readlines()]`