Why I am not able to run rails tests

Question

This is what I did.

> git clone git://github.com/rails/rails.git
> cd rails
> cd railties
> rake

And I got following error.

(in /Users/dorelal/dev/scratch/rails/railties)
./test/isolation/abstract_unit.rb:236:in `initialize': No such file or directory - /Users/dorelal/dev/scratch/rails/railties/tmp/app_template/config/boot.rb (Errno::ENOENT)
    from ./test/isolation/abstract_unit.rb:236:in `open'
    from ./test/isolation/abstract_unit.rb:236
    from ./test/isolation/abstract_unit.rb:222:in `initialize'
    from ./test/isolation/abstract_unit.rb:222:in `new'
    from ./test/isolation/abstract_unit.rb:222
    from test/application/configuration_test.rb:1:in `require'
    from test/application/configuration_test.rb:1
rake aborted!

I checked ~/railties/tmp and this directory is empty.

I know rails is not broken. So what am I missing?