Clean up find_by_name test
Test find_by_name with an active prerelease

This shows that the behavior described in #988 doesn't match the
behavior of RubyGems.  According to my reading, @niedfelj expects
find_by_name to return a prerelease version if it is active.  This test
successfully finds such a gem.

Bundler should be marking the gem as active for RubyGems to find as
well, so perhaps the problem lies there.

Maybe there is a different issue in RubyGems that we have yet to
discover.