make/testing_flags: delete log files before running itests, print date
make/testing_flags: delete log files before running itests, print date

This commit makes sure old log files are deleted before running the
integration tests. Previously new logs would be added to the existing
ones, causing a big mess. This was especially messy during flake
hunting, as all logs would be kept.

In addition, we now print the date before every start of the integration
tests, which is useful to see how long they have been running, for
instance in case of a deadlock.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd