Exclude C API MRI test with non-standard extension name

* The extension should be in test/mri/tests/cext-c/array/to_ary_conact
  but is in test/mri/tests/cext-c/array/concat

Author: eregon

test/mri/failing.exclude

@@ -36,6 +36,7 @@ racc/test_racc_command.rb
 
 # Other issues
 
+cext-ruby/array/test_to_ary_concat.rb # non-standard extension name and irrelevant
 cext-ruby/bignum/test_big2str.rb
 cext-ruby/bignum/test_bigzero.rb
 cext-ruby/bignum/test_div.rb