tests: fix tail test race causing false failure

tests: fix tail test race causing false failure

* tests/tail-2/retry.sh: The replacement of the "missing" directory
is not atomic, and therefore tail(1) can take a different path,
especially if there is a delay between the rmdir(2) and creat(2).
This is noticeable for example with `make coverage` because in
that case the coverage files written by rmdir(1) on exit,
induce a significant delay thus triggering the issue.