test-example: Generate new style sites.conf

The key change here is to make the `normal' test-exampple/sites.conf
file be new-style, with peer-keys entries.

We also switch the tests to use the old-style file.  So the tests in
fact run just as before.

The directory we give to make-secnet-sites is already mkdir'd, and
cleaned, appropriately.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>