[ruby/irb] Use "csv" for test

The "csv" doesn't conflict with other stdlib names in any Ruby processing
system.

https://github.com/ruby/irb/commit/49203a1c83

test/irb/test_completion.rb

@@ -69,8 +69,8 @@ module TestIRB
     end
 
     def test_complete_require_library_name_first
-      candidates = IRB::InputCompletor::CompletionProc.("'cgi", "require ", "")
-      assert_equal candidates.first, "'cgi"
+      candidates = IRB::InputCompletor::CompletionProc.("'csv", "require ", "")
+      assert_equal candidates.first, "'csv"
     end
 
     def test_complete_require_relative