Use secure password min cost option in its own tests for a speed up

Around 0.564359s => 0.092244s speed up in my machine.

activemodel/test/cases/secure_password_test.rb

@@ -5,13 +5,18 @@ require 'models/visitor'
 require 'models/administrator'
 
 class SecurePasswordTest < ActiveModel::TestCase
-
   setup do
+    ActiveModel::SecurePassword.min_cost = true
+
     @user = User.new
     @visitor = Visitor.new
     @oauthed_user = OauthedUser.new
   end
 
+  teardown do
+    ActiveModel::SecurePassword.min_cost = false
+  end
+
   test "blank password" do
     @user.password = @visitor.password = ''
     assert !@user.valid?(:create), 'user should be invalid'
@@ -70,13 +75,16 @@ class SecurePasswordTest < ActiveModel::TestCase
     end
   end
 
-  test "Password digest cost defaults to bcrypt default cost" do
+  test "Password digest cost defaults to bcrypt default cost when min_cost is false" do
+    ActiveModel::SecurePassword.min_cost = false
+
     @user.password = "secret"
     assert_equal BCrypt::Engine::DEFAULT_COST, @user.password_digest.cost
   end
 
   test "Password digest cost can be set to bcrypt min cost to speed up tests" do
     ActiveModel::SecurePassword.min_cost = true
+
     @user.password = "secret"
     assert_equal BCrypt::Engine::MIN_COST, @user.password_digest.cost
   end