fix traceback in useless i386 tests check

The recent change in commit ac6b0c1ba7 is
not currently functional - it's hard to test and although similar
functionality exists elsewhere in the code, the changes in said commit
resulted in the following traceback:
```
'str' object has no attribute 'architecture'
```

This commit amends the issue by utilising the `binaries_info` variable
instead, with its properties defined here:
https://git.launchpad.net/~ubuntu-release/britney/+git/britney2-ubuntu/tree/britney2/__init__.py#n304

Brian had previously made this commit:
https://git.launchpad.net/~ubuntu-release/britney/+git/britney2-ubuntu/commit/?id=663bd2571cc81a9477408c0711aeb6b2452c17a8
to try and amend the issue, though it seems it hasn't helped.

This commit also adds a little bit more logging surrounding this new
functionality and also adds a break statement for efficiency.

britney2/policies/autopkgtest.py

@@ -705,15 +705,19 @@ class AutopkgtestPolicy(BasePolicy):
         try:
             if arch == "i386":
                 all_binaries_arch_all = True
-                srcinfo = source_suite.sources[src]
-                for pkg_id in srcinfo.binaries:
-                    if pkg_id.architecture != "all":
+                for package_name in binaries_info.keys():
+                    if binaries_info[package_name].architecture != "all":
                         all_binaries_arch_all = False
+                        break
                 if all_binaries_arch_all:
                     self.logger.info('Source package %s has binaries which are all Architecture: all, and tests have been requested on %s, not running any tests for this src package',
                                      src,
                                      arch)
                     return []
+                else:
+                    self.logger.info('Source package %s has binaries which are NOT Architecture: all, and tests have been requested on %s, running tests for this package',
+                                     src,
+                                     arch)
         except Exception as e:
             self.logger.error('i386 useless autopkgtest check failed with: %s', e)