Refactor AR's validations_test.rb

The `:context` switch feature is implemented in
ActiveRecord::Validations#valid? method, so we should rename the test
names, and execute `valid?` in the test.

Change test name in AR's validations_test.rb

This test is testing save method's code
Merge pull request #10662 from take/change-test-name-for-ActiveRecord--Validations#valid-

Refactor AR's validations_test.rb