Fix build about TemplateDigestorTest.

[kennyj]

I found the following test failure.

  1) Error:
143test_collection_dependency(TemplateDigestorTest):
144Errno::ENOENT: No such file or directory - /home/vagrant/builds/rails/rails/actionpack/test/template/../tmp/messages/_message.html.erb
145    /home/vagrant/builds/rails/rails/actionpack/test/template/digestor_test.rb:153:in `initialize'
...

This problem came from FiltUtils.cp_r(src, dest) specific.
According to API reference, if dest is directory, files were copied to dest/src directory.
We copy files in setup, and remove directory in teardown.
Thus if we execute tests twice, tests are failure at first, and tests are success at second.