tests: avoid a race in tail --retry testing

tests: avoid a race in tail --retry testing

Prompted by the continuous integration build failure at:
http://hydra.nixos.org/build/5221053

* tests/tail-2/retry.sh: Ensure the 'out' file is truncated,
as it's used to arbitrate the run order of commands.
Relying on the truncation in the background tail command
is racy because the truncation can occur after the fork
of the background shell and thus wait4lines would not wait
for output to occur in 'out', which would mean that the
'missing' file could be populated by the time tail(1)
gets to process it.