py3: use sysstr() to convert ProgrammingError bytes with no unicode error risk
msg.decode('utf8') may fail if msg isn't an ASCII string, and that's possible
as we sometimes embed a filename in the error message for example.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL