Bug#1041336: fenix-plugins: build improvements

[Thomas Uhle]

Dear maintainers,

yesterday I forgot to attach a patch that let autopkgtest run the tests 
just on the 32-bit architectures for which the packages fenix-plugins, 
fenix-plugins-system and fenix-plugin-mpeg are built to avoid the current 
test failures on the 64-bit architectures [1].

Best regards,

Thomas Uhle


[1] https://ci.debian.net/packages/f/fenix-plugins