use variables from test setup

activemodel/test/cases/secure_password_test.rb

@@ -10,15 +10,13 @@ class SecurePasswordTest < ActiveModel::TestCase
   end
 
   test "blank password" do
-    user = User.new
-    user.password = ''
-    assert !user.valid?, 'user should be invalid'
+    @user.password = ''
+    assert !@user.valid?, 'user should be invalid'
   end
 
   test "nil password" do
-    user = User.new
-    user.password = nil
-    assert !user.valid?, 'user should be invalid'
+    @user.password = nil
+    assert !@user.valid?, 'user should be invalid'
   end
 
   test "password must be present" do