Fix Separate test cases for RSpec/ChangeByZero cop

These examples were incorrectly grouped under
"by is not zero" test cases despite testing different behavior.

Author: rito.ishihara

spec/rubocop/cop/rspec/change_by_zero_spec.rb

@@ -365,10 +365,11 @@
         expect { foo }.to change { Foo.bar }.by(1)
       end
     RUBY
+  end
 
+  it 'does not register an offense when without expect block' do
     expect_no_offenses(<<~RUBY)
       it do
-        subject; change(Foo, :bar).by(0)
         change(foo, :bar).by(0)
       end
     RUBY