Call clean.sh from all relevant setup.sh scripts

For all system tests utilizing named instances, call clean.sh from each
test's setup.sh script in a consistent way to make sure running the same
system test multiple times using run.sh does not trigger false positives
caused by stale files created by previous runs.

Ideally we would just call clean.sh from run.sh, but that would break
some quirky system tests like "rpz" or "rpzrecurse" and being consistent
for the time being does not hurt.

bin/tests/system/stress/setup.sh

@@ -9,6 +9,11 @@
 # See the COPYRIGHT file distributed with this work for additional
 # information regarding copyright ownership.
 
+# shellcheck source=conf.sh
+. "$SYSTEMTESTTOP/conf.sh"
+
+$SHELL clean.sh
+
 #
 # Set up test data for zone transfer quota tests.
 #