masher: Find the old updateinfo files

Now that we prefix the updateinfo metadata with its hash, the Bodhi
masher must be taught to find it.

The bonus point of using a glob is that it even handles migration from
non-prefixed to prefixed file name. :)
tests: Teach the metadata tests about the new updateinfo name
tests: Add a message on the assertion error

This assertion was failing due to a syntax error in my masher/updateinfo
code.

Adding this message allowed me to find it quickly, and it could be
useful in the future.