add an additional test for arch specific conflicts

In bugreport #747261 I confirmed with this testcase that apt actually
supports the requested architecture-specific conflicts already since
2012 with commit cef094c2ec8214b2783a2ac3aa70cf835381eae1.

The old test only does simulations which are handy to check apt,
this one builds 'real' packages to see if dpkg agrees with us.

Git-Dch: Ignore