Doesn't install crossbuild-essential-$arch unless the test depends on build-essential

We have a delta on our libinih package due to that issue. It was discussed on https://irclogs.ubuntu.com/2021/02/25/%23ubuntu-devel.html#t20:26

I don't know if that's still an actual problem but I'm at least reporting a bug to document the workaround delta we carry in the package.

Relevant quote from Steve on IRC
> apparently what I implemented pulls in crossbuild-essential-i386 only if 'build-essential' is explicitly
> listed as a test dep; this seems to be a bug in my autopkgtest implementation, since @builddeps@ does
> imply build-essential:native but does not cause crossbuild-essential-i386 to be pulled in.
> so, can be worked around by adding build-essential to the test deps, but ultimately I should fix
> the autopkgtest implementation