test-worker: exercise more about "killworkers" situation

This patch adds some sleep and increases numcpus to exercise the
"killworkers" situation. So race conditions could be discovered more easily,
if someone changes worker.py incorrectly. Currently worker.py should be free
of such issues.

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 time
   > from mercurial import (
   >     cmdutil,
   >     error,
@@ -22,6 +23,7 b' Test UI worker interaction'
   >     for arg in args:
   >         ui.status('run\n')
   >         yield 1, arg
+  >     time.sleep(0.1) # easier to trigger killworkers code path
   > functable = {
   >     'abort': abort,
   >     'exc': exc,
@@ -74,20 +76,20 b' Run tests without worker by forcing a lo'
 
 Known exception should be caught, but printed if --traceback is enabled
 
-  $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=2' \
-  > test 100000.0 abort
+  $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=8' \
+  > test 100000.0 abort 2>&1
   start
   abort: known exception
   [255]
 
-  $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=2' \
+  $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=8' \
   > test 100000.0 abort --traceback 2>&1 | egrep '^(SystemExit|Abort)'
   Abort: known exception
   SystemExit: 255
 
 Traceback must be printed for unknown exceptions
 
-  $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=2' \
+  $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=8' \
   > test 100000.0 exc 2>&1 | grep '^Exception'
   Exception: unknown exception