Follow renaming from Namespace to Ruby::Box

Author: tagomoris

test/-ext-/test_abi.rb

@@ -9,7 +9,7 @@ def test_require_lib_with_incorrect_abi_on_dev_ruby
     assert_separately [], <<~RUBY
       err = assert_raise(LoadError) { require "-test-/abi" }
       assert_match(/incompatible ABI version/, err.message)
-      if Namespace.enabled?
+      if Ruby::Box.enabled?
         assert_include err.message, "_-test-+abi."
       else
         assert_include err.message, "/-test-/abi."
@@ -31,7 +31,7 @@ def test_enable_abi_check_using_environment_variable
     assert_separately [{ "RUBY_ABI_CHECK" => "1" }], <<~RUBY
       err = assert_raise(LoadError) { require "-test-/abi" }
       assert_match(/incompatible ABI version/, err.message)
-      if Namespace.enabled?
+      if Ruby::Box.enabled?
         assert_include err.message, "_-test-+abi."
       else
         assert_include err.message, "/-test-/abi."