test-worker: capture tracebacks more reliably

The traceback test may have traceback caused by SIGTERM. Instead of grepping
"Traceback", explicitly grep the exception we care about.

This makes the test less flaky.

tests/test-worker.t

@@ -81,15 +81,15 b' Known exception should be caught, but pr'
   [255]
 
   $ hg --config "extensions.t=$abspath" --config 'worker.numcpus=2' \
-  > test 100000.0 abort --traceback 2>&1 | grep '^Traceback'
-  Traceback (most recent call last):
-  Traceback (most recent call last):
+  > 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' \
-  > test 100000.0 exc 2>&1 | grep '^Traceback'
-  Traceback (most recent call last):
+  > test 100000.0 exc 2>&1 | grep '^Exception'
+  Exception: unknown exception
 
 Workers should not do cleanups in all cases