Promote 'testbn' to a binary built by the makefiles.

This makes it easier to compile in multiple debugging modes, or on
Windows, without having to constantly paste annoying test-compile
commands out of comments in sshbn.c.

The new binary is compiled into the build directory, but not shipped
by 'make install', just like fuzzterm. Unlike fuzzterm, though, testbn
is also compiled on Windows, for which we didn't already have a
mechanism for building unshipped binaries; I've done the very simplest
thing for the moment, of providing a target in Makefile.vc to delete
them.

In order to comply with the PuTTY makefile system's constraint of
never compiling the same object multiple times with different ifdefs,
I've also moved testbn's main() out into its own source file.