enforcesinglehead-test: add the expected node output next to the error

this make it simpler to validate that the test is correct.

Differential Revision: https://phab.mercurial-scm.org/D10545

tests/test-single-head.t

@@ -65,6 +65,9 b' Create a new head on the default branch'
   1 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ mkcommit c_dD0
   created new head
+  $ hg log -r 'heads(::branch("default"))' -T '{node|short}\n'
+  286d02a6e2a2
+  9bf953aa81f6
   $ hg push -f
   pushing to $TESTTMP/single-head-server
   searching for changes