test: make sure we hit the SIGPIPE in test-transaction-rollback-on-sigpipe

With a coming change, we might not hit the sig pipe without issue and extra line
of output. We do this early to make the next change clearer.

Differential Revision: https://phab.mercurial-scm.org/D10946

tests/test-transaction-rollback-on-sigpipe.t

@@ -47,7 +47,8 b' disconnecting. Then exit nonzero, to for'
 
   $ cat >remote/.hg/hgrc <<EOF
   > [hooks]
-  > pretxnchangegroup.break-things=$hook_script
+  > pretxnchangegroup.00-break-things=$hook_script
+  > pretxnchangegroup.01-output-things=echo "some remote output to be forward to the closed pipe"
   > EOF
 
   $ hg --cwd ./remote tip -T '{node|short}\n'