Fix flaky require test

Author: luke-gru

test/ruby/test_require.rb

@@ -857,7 +857,13 @@ def to_str
         def to_path = @path
       end
 
-      def create_ruby_file = Tempfile.open(["test", ".rb"]).path
+      FILES = []
+
+      def create_ruby_file
+        file = Tempfile.open(["test", ".rb"])
+        FILES << file
+        file.path
+      end
 
       require MyString.new(create_ruby_file)
       $LOADED_FEATURES.unshift(create_ruby_file)