Require "active_support/core_ext/object/blank" for railties abstract unit
Require "active_support/core_ext/object/blank" for railties abstract unit

In #25380 I moved an `active_support/core_ext/object/blank` in the
`activesupport/lib/active_support/testing/assertions.rb`, however it caused a
chain reaction, resulting in failing railties tests.

Moving the require to `railties/test/isolation/abstract_unit.rb` and
`activesupport/test/abstract_unit.rb`.

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