What happens when "rake test"?

I meet a problem which become a stopper. When I run “rake test” .
then there always comes the error :

D:/InstantRails/ruby/lib/ruby/1.8/csv.rb:11: CSV is not a class

my enviroment is Rails1.2.1+Ruby1.8.4. The same issue can be
reproduced in the same code in Rails1.2.2+Ruby1.8.5

currently i can comments this then all works.

\fixtures.rb (line 3)
require ‘csv’

but what i think this is a damage to the project.

what I am interested is , what happens when i run “rake test”? i
would like to debug into the source code to see what is going on and
why there is such bug.