To hopefully shut up CodeQL "Iterable can be either a string or a sequence".
This change makes the code more readable anyway, so it is more than just
a gratuitous change to make some code-scanner happy.
Use universalasync library to make test.py async code compatible
with synchronous code of dtest/ccm
Also, copied unmodified error_example_test.py from dtest as an example.
Run the test in `dev` mode only.