Fix WMBSMergeBySize causing duplicates

This occurs when a site has multiple SEs, fix
changes the discovery query to return a file only once
with a tuple of the possible SEs.

@sfoulkes please review, same problem as in f222ab2f2ccf82a6f8539d78c4590cb2171a6e92. Here the fix is different to mantain the parent file boundaries. Unit tests were modified to fail without the fixes.