Make activemodel tests runnable without -Itest and from non-root folders

[grosser]

I try to make testing as easy as possible, which means for me that every test file should be runnable with ruby pat/to/file_test.rb from everywhere, which is also a requirement for a few editors, this failed in activemodel.
(I'll make more pull requests for the other rails parts if this one is accepted)

[steveklabnik]

This does not merge cleanly any more, and will need a rebase.

[grosser]

@steveklabnik do I get a merge if I do ?

[steveklabnik]

I'm not allowed to merge code. :( I personally like it.

[grosser]

@josevalim would you merge it ?

[rafaelfranca]

@grosser thank you for the pull request.

For me is not a problem writing -Itest and I don't see the point of changing this since it will make more difficult to add new test files because we have to take care with the relativity of the folders.