delete duplicated tests in AR base_test.rb

After I opened #9312, I realized that these tests are unnecessary.

These are duplicated with the tests which are in

- [persistence_test.rb](https://github.com/rails/rails/blob/master/activerecord/test/cases/persistence_test.rb#L167) ( test_create_~
- [relations_test.rb](https://github.com/rails/rails/blob/master/activerecord/test/cases/relations_test.rb#L946) ( test_first_or_~

Thx