Change the test name in validations_test.rb to a more specific name

I was looking tests for the `ActiveRecord::Validations#valid?` method, and it was hard to find out this test.
IDK if the diff `_to_valid?` is really a good way of adding the method name, since tests for method like `save!` and `create!` are like `test_create_bang~`, but `test_valid_question~` seems kinda too long... :(