New test program 'test_conf', mostly transitional.

This aims to be a reasonably exhaustive test of what happens if you
set Conf values to various things, and then save your session, and
find out what ends up in the storage. Or vice versa.

Currently, the test program is written to match the existing
behaviour. The idea is that I can refactor the code that does the
loading and saving, and if this test still passes, I've probably done
it right.

However, in the long term, this test will be a liability: it's yet
another place you have to add every new config option. So my plan is
to get rid of it again once the refactorings I'm planning are
finished.

Or rather, I'll get rid of _that_ part of its functionality. I also
suspect I'll have added new kinds of consistency check by then, which
won't be a liability in the same way, and which I'll want to keep.

Buildscr

@@ -142,6 +142,7 @@ in putty do make -j$(nproc) VERBOSE=1
 in putty do python3 test/cryptsuite.py
 in putty do ./test_lineedit
 in putty do ./test_terminal
+in putty do ./test_conf
 enddelegate
 
 delegate -