mtest: Use --pubkeys-elide instead of --output-version

We don't want our basic tests of make-secnet-sites to produce an
artificially old output version.

But also it's not really convenient for these tests to make a pubkeys
directory.  (That functionality is tested by test-example's sites.conf
generator and stest's uses of the resulting pubkeys directory.)

So use --pubkeys-elide, conveniently producing a test case for that
option, which we were previously lacking.

Because we don't pass --pubkeys-dir, the result is that the output
paths contain the default pubkeys directory.  That's OK.

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