Make base fixture for testitems, remove multi-app inventory tests

Since every other valve game only uses a subset of TF2's schema
and item related features there is really no point in looping through
various other games. It will quickly add up and make the unit tests take
too long to run. For this reason I instead made a base class containing
the app ID and ID64 to be tested against so it can be adjusted as
needed.
Add name and attribute tests
Fix quality prefix being stripped on non-unique items