py3: suppress unraisable exceptions in test-worker.t

Python 3.8 calls sys.unraisablehook when an unraisable
exception is encountered. The default behavior is to print a
warning.

test-worker.t was triggering this hook due to a race between
a newly forked process exiting and that process's
_os.register_at_fork handlers running. I was seeing the
stdlib's random module in the stack re-seeding itself. Although
there could be other after-fork handlers in the mix.

This commit defines sys.unraisablehook to effectively no-op.
This suppresses the warning and makes test output on Python 3.8
consistent with prior versions. test-worker.t now passes on
Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D7949

tests/test-worker.t

@@ -2,6 +2,7 b' Test UI worker interaction'
 
   $ cat > t.py <<EOF
   > from __future__ import absolute_import, print_function
+  > import sys
   > import time
   > from mercurial import (
   >     error,
@@ -9,6 +10,7 b' Test UI worker interaction'
   >     ui as uimod,
   >     worker,
   > )
+  > sys.unraisablehook = lambda x: None
   > def abort(ui, args):
   >     if args[0] == 0:
   >         # by first worker for test stability
@@ -101,7 +103,9 b' Workers should not do cleanups in all ca'
   > from __future__ import absolute_import
   > import atexit
   > import os
+  > import sys
   > import time
+  > sys.unraisablehook = lambda x: None
   > oldfork = os.fork
   > count = 0
   > parentpid = os.getpid()