* stderr/stdout: Do not assume well-formed UTF-8 output (Arne Welzel, Corelight)
The previous code opened stdout/stderr providing an explicit encoding="utf-8"
argument. This has no effect when the file handles are passed as stderr/stdout
to a subprocess: Only the underlying fds are passed and there's no way to
control the encoding (if any) used by the subprocess for the bytes it
produces on these streams.
So, switch things around: Remove the misleading encoding arguments for the
stdout, stderr files, open them in binary mode and be careful when
opening .stderr for displaying purposes. Do try UTF-8, but also set error
strategy to "backslashreplace" to avoid errors.
* monitor_cb: Start thread with daemon=True (Arne Welzel, Corelight)
When an unexpected exception happened in a child process, nothing would
stop the monitor_cb thread and in turn the child process would never
finish. Use daemon=True on the monitor_cb thread to prevent this.
There should never be unhandled exceptions in child processes, because
it may then not close all resources properly etc, but a non-terminating
btest is worse, specifically when btest is run in CI unattended.