test-example: Test new long-term key arrangements

Finally we can test this!

Specifically, we switch the defaults to the privkeys directories,
which affects the existing t-basic-kex test.  We introduce two new
tests for testing the old arrangements.

The consequences for the t-basic-kex test are:
 * Use the sites file generated with --install-pubkeys, thus
   testing the new make-secnet-sites.
 * Use peer-keys, so testing secnet's new pubkey loading
   mechanisms.
 * Use priv-cache, so testing secnet's new privkey loading
   mechanisms.
 * Test the new public key choice negotation mechanism.

The old tests tests the compatibility of the key choice negotiation
mechanism, as well as the capability to generate
old-style (compatible) sites.conf files.

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