test: Count writes to stderr as failures

test: Count writes to stderr as failures

Python 3 logs warnings in destructors if objects responsible for file
descriptors are not explicitly closed. These warnings were previously
being ignored by our test suite unless a human was looking at the log
output, but several recent PRs have introduced these issues. This
change ensures we catch them (and fixes the most recent one in
process_test. The leak has always been there, but the previous commit
caused it to be logged).