Rails test abstrct_unit

anyone help? When I try to test my file, I always get this:

macworld@macworld-virtual-machine:~/rails/activerecord$ ruby -Itest
test/cases/type_cast_test.rb
/home/macworld/.rvm/rubies/ruby-1.9.3-p362/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in
require': cannot load such file -- abstract_unit (LoadError) from /home/macworld/.rvm/rubies/ruby-1.9.3-p362/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:inrequire’
from test/cases/type_cast_test.rb:1:in `’
macworld@macworld-virtual-machine:~/rails/activerecord$