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
hgterm,
am, km, mir, msgr, xenl,
colors#8, cols#80, it#8, lines#24, pairs#64,
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