Remove code related to the rails test command

Hello,

I've recently upgrade to Rails 4.0 and seen #9080 several months ago. Spontaneously, I run `rails test` but got : 

~~~
Error: Command 'test' not recognized
...
 test        Running the test file (short-cut alias: "t")
~~~

This pull request removes the mention about this command in the rails command's help message and remove the "t" alias related to it.

If you want, I can backport it to 4-0-0 and 4-0-0-stable as well ; let me know.

Also I don't add any changelog entry but let me know if I should.

Have a nice day.