Prevent a warning: method redefined; discarding old included

```
[ 3463/25719] JSONGeneratorTest#test_string_ext_included_calls_super/home/chkbuild/chkbuild/tmp/build/20230905T063003Z/ruby/test/json/json_generator_test.rb:411: warning: method redefined; discarding old included
/home/chkbuild/chkbuild/tmp/build/20230905T063003Z/ruby/test/json/json_generator_test.rb:399: warning: previous definition of included was here
```

http://rubyci.s3.amazonaws.com/debian10/ruby-master/log/20230905T063003Z.log.html.gz

tests/json_generator_test.rb

@@ -396,6 +396,7 @@ def test_string_ext_included_calls_super
       included = false
 
       Module.alias_method(:included_orig, :included)
+      Module.remove_method(:included)
       Module.define_method(:included) do |base|
         included_orig(base)
         included = true
@@ -408,6 +409,7 @@ def test_string_ext_included_calls_super
       assert included
     ensure
       if Module.private_method_defined?(:included_orig)
+        Module.remove_method(:included) if Module.method_defined?(:included)
         Module.alias_method(:included, :included_orig)
         Module.remove_method(:included_orig)
       end