test_settracefunc.rb: fix tests

* test/ruby/test_settracefunc.rb (assert_security_error_safe4): fix
  tests to set $SAFE separatedly

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38972 b2dd03c8-39d4-4d8f-98ff-823fe69b080e