autopkgtest: add a must-fail test
The existing must-succeed test protects against potential bugs that make piuparts always fail. Other potential bugs might make piuparts always succeed, so let's add a must-fail test to cover this case, too.
The existing must-succeed test protects against potential bugs that make piuparts always fail. Other potential bugs might make piuparts always succeed, so let's add a must-fail test to cover this case, too.