In case invoke_on_all(tester::start) throws, the sharded<tester>
instance remains non-stopped and calltrace is reported on test stop. Not
nice, fix it so that sharded<> thing is stopped in any case.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closesscylladb/scylladb#23244