tests: avoid rare race in tail-2/F-vs-rename.sh

tests: avoid rare race in tail-2/F-vs-rename.sh

* tests/tail-2/F-vs-rename.sh: Keep stdout and stderr separate,
so that interspersion doesn't impact regex checks.  Also wait
for each file's data to be printed to avoid multiple writes
to a file to be printed in a single iteration, which would
impact the regex checks.  Also we refactor the check function,
rather than repeatedly redefining variations.