topic/concurrency_testfixes

Cosmetic fixes for some tests. Don't know what is the policy for direct-to-master commits, so opening a PR in case anyone wants to look at it.

Testing methodology: 
```
bash -c '
(perl Makefile.PL && make realclean && perl Makefile.PL && make)&>/dev/null || exit 125
perl -v
for n in $(seq 1000); do
  echo -n . && OUT=$(! prove -s -r -b -j32 2>&1) && EX=1 && echo "$OUT" && break
done
echo
exit "${EX:-0}"
' && echo ok
```