testrunner: use "#" for "test cases" suffix in .err filename too
This seems like a natural follow-up to b865bba56db1 (run-tests: update
the test case name format, 2018-05-13).
Differential Revision: https://phab.mercurial-scm.org/D4052
#require docutils gettext $ $TESTDIR/check-gendoc fr checking for parse errors