Cleaning up test helpers related to OmniAuth.

Developers should rely on OmniAuth's new testing API. Check
https://github.com/intridea/omniauth/wiki/Integration-Testing
for more details.

test/omniauth/test_helpers_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+class OmniAuthTestHelpersTest < ActiveSupport::TestCase
+  test "Assert that stub! raises deprecation error" do
+    assert_raises Devise::OmniAuth::TestHelpers::DeprecationError do
+      Devise::OmniAuth::TestHelpers.stub!
+    end
+  end
+
+  test "Assert that reset_stubs! raises deprecation error" do
+    assert_raises Devise::OmniAuth::TestHelpers::DeprecationError do
+      Devise::OmniAuth::TestHelpers.reset_stubs!
+    end
+  end
+
+  test "Assert that short_circuit_authorizers! warns about deprecation" do
+    Devise::OmniAuth::TestHelpers.short_circuit_authorizers!
+    assert ::OmniAuth.config.test_mode
+  end
+
+  test "Assert that unshort_circuit_authorizers! warns about deprecation" do
+    Devise::OmniAuth::TestHelpers.unshort_circuit_authorizers!
+    assert ! ::OmniAuth.config.test_mode
+  end
+end