DEV: Properly cleanup auth providers in test (#24482)

Followup to 5c38e55dc9

spec/requests/omniauth_callbacks_controller_spec.rb

@@ -1125,13 +1125,15 @@ RSpec.describe Users::OmniauthCallbacksController do
       end
     end
 
+    let(:fake_auth_provider) { Auth::AuthProvider.new(authenticator: FakeAuthenticator.new) }
+
     before do
-      DiscoursePluginRegistry.register_auth_provider(
-        Auth::AuthProvider.new(authenticator: FakeAuthenticator.new),
-      )
+      DiscoursePluginRegistry.register_auth_provider(fake_auth_provider)
       OmniAuth.config.test_mode = false
     end
 
+    after { DiscoursePluginRegistry.auth_providers.delete(fake_auth_provider) }
+
     it "does not run 'other_phase' for disabled auth methods" do
       get "/auth/fake/blah"
       expect(response.status).to eq(404)