Issue #16762: Fix some test_subprocess failures on NetBSD and OpenBSD: kill()
returns ESRCH for a zombie process, which is not POSIX-compliant.
Issue #16829: IDLE printing no longer fails if there are spaces or other
special characters in the file path.
test_asyncore: wait explicitly for a thread termination (this dangling thread
could be the cause of a random failure).
Issue #16922: fixed findtext() to return empty Unicode string instead of empty bytes object when there's no text.

Patch by Serhiy Storchaka.