stest: Support not running secnet ourselves

stest: Support not running secnet ourselves

Now the user can set SECNET_STEST_DIVERT_inside=1 (say) and instead of
actually running secnet, we print the rune we would run and ask the
user when they have started it.

A freshly started secnet (with this config, at least) just sits there
waiting for its first packet to deal with.  So if the user does this
for the 2nd secnet, that's fine - the first one will happily wait
until the test actually gets going.

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