Currently, executing the test with only `attribute_set_test.rb` results in an error.
```
./bin/test -w test/cases/attribute_set_test.rb
Run options: --seed 33470
# Running:
E
Error:
ActiveModel::AttributeSetTest#test_#map_returns_a_new_attribute_set_with_the_changes_applied:
NameError: uninitialized constant ActiveModel::AttributeSetTest::AttributeSet
Did you mean? ActiveModel::Attributes
ActiveModel::Attribute
activemodel/test/cases/attribute_set_test.rb:235:in `block in <class:AttributeSetTest>'
bin/test test/cases/attribute_set_test.rb:234
```
Added a missing require to fix this.
Also, I suspect that this is the cause of failures in CI.
Ref: https://travis-ci.org/rails/rails/jobs/299994708