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
adding changesets
adding manifests
adding file changes
added 1 changesets with 4 changes to 4 files
new changesets d326ae2d01ee
4 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding changesets
adding manifests
adding file changes
added 1 changesets with 4 changes to 4 files
new changesets d326ae2d01ee
4 files updated, 0 files merged, 0 files removed, 0 files unresolved