Use untaint for File.symlink in kernel_require.rb

Partly reverted 7d463e360b

lib/rubygems/core_ext/kernel_require.rb

@@ -41,7 +41,10 @@ module Kernel
     resolved_path = begin
       rp = nil
       $LOAD_PATH[0...Gem.load_path_insert_index || -1].each do |lp|
-        safe_lp = lp.dup.tap(&Gem::UNTAINT)
+        # TODO:
+        # for test_require.rb of ruby core test suite
+        # We should use Gem::UNTAINT after https://bugs.ruby-lang.org/issues/16131
+        safe_lp = lp.dup.untaint
         begin
           if File.symlink? safe_lp # for backword compatibility
             next