Use expect_correction instead of autocorrect_source_with_loop

Follow up #8062.

This commit fixes the following build error because
`autocorrect_source_with_loop` has been removed by
rubocop/rubocop@510f3f7f.

```console
% bundle exec rake
(snip)

  1) RuboCop::Cop::Performance::CaseWhenSplat autocorrect moves multiple
  splat condition to the end of the when conditions
     Failure/Error:
       new_source = autocorrect_source_with_loop(<<~RUBY)
         case foo
         when *cond1
           bar
         when *cond2
           foobar
         when 5
           baz
         end
       RUBY

     NoMethodError:
       undefined method `autocorrect_source_with_loop' for
       #<RSpec::ExampleGroups::RuboCopCopPerformanceCaseWhenSplat::Autocorrect:0x00007f881ec9df58>
       Did you mean?  autocorrect_source_file
     # ./spec/rubocop/cop/performance/case_when_splat_spec.rb:277:in
       `block (3 levels) in <top (required)>'

  2) RuboCop::Cop::Performance::CaseWhenSplat autocorrect moves multiple
  out of order splat condition to the end of the when conditions
     Failure/Error:
       new_source = autocorrect_source_with_loop(<<~RUBY)
         case foo
         when *cond1
           bar
         when 3
           doo
         when *cond2
           foobar
         when 6
           baz

     NoMethodError:
       undefined method `autocorrect_source_with_loop' for
       #<RSpec::ExampleGroups::RuboCopCopPerformanceCaseWhenSplat::Autocorrect:0x00007f881ec2df00>
       Did you mean?  autocorrect_source_file
     # ./spec/rubocop/cop/performance/case_when_splat_spec.rb:302:in
       `block (3 levels) in <top (required)>'
```

spec/rubocop/cop/performance/case_when_splat_spec.rb

@@ -274,7 +274,7 @@
     end
 
     it 'moves multiple splat condition to the end of the when conditions' do
-      new_source = autocorrect_source_with_loop(<<~RUBY)
+      new_source = autocorrect_source(<<~RUBY)
         case foo
         when *cond1
           bar
@@ -299,7 +299,7 @@
 
     it 'moves multiple out of order splat condition to the end ' \
        'of the when conditions' do
-      new_source = autocorrect_source_with_loop(<<~RUBY)
+      new_source = autocorrect_source(<<~RUBY)
         case foo
         when *cond1
           bar