bliss: Fix monolithic build

This requires moving test files so that the Makefile for the tests can be
included after building libstrongswan, which requires the plugin when
building monolithically.  Due to this a static helper library is required
as directly referring to object files (or source files) is not possible.

It's also necessary to avoid any link-time dependency on libstrongswan in
bliss_huffman, to avoid circular dependencies (bliss_huffman -> libstrongswan
-> bliss -> bliss_huffman).