Support request specs in the sign_in_as helper

I've updated the `sign_in_as` test helper method so it can be used for
both system specs and request specs.

Since they're both operating under different contexts, authentication
needs to be mocked differently depending on the type of test.

spec/requests/placements/providers/users_spec.rb

@@ -5,9 +5,7 @@
   let(:current_user) { create(:placements_user, providers: [provider]) }
 
   before do
-    user_exists_in_dfe_sign_in(user: current_user)
-    get "/auth/dfe/callback"
-    follow_redirect!
+    sign_in_as current_user
   end
 
   describe "DELETE" do

spec/requests/placements/schools/placements/placements_spec.rb

@@ -6,11 +6,7 @@
 
     before do
       user = create(:placements_user, schools: [school])
-      user_exists_in_dfe_sign_in(user:)
-
-      get "/auth/dfe/callback"
-
-      follow_redirect!
+      sign_in_as user
     end
 
     context "when editing mentors" do

spec/requests/placements/schools/users_spec.rb

@@ -5,9 +5,7 @@
   let(:current_user) { create(:placements_user, schools: [school]) }
 
   before do
-    user_exists_in_dfe_sign_in(user: current_user)
-    get "/auth/dfe/callback"
-    follow_redirect!
+    sign_in_as current_user
   end
 
   describe "DELETE" do

spec/requests/placements/support/support_users_spec.rb

@@ -5,9 +5,7 @@
   let(:support_users) { Placements::SupportUser.all }
 
   before do
-    user_exists_in_dfe_sign_in(user: current_user)
-    get "/auth/dfe/callback"
-    follow_redirect!
+    sign_in_as current_user
   end
 
   describe "DELETE" do

spec/requests/placements/support/user_authorisation_spec.rb

@@ -55,10 +55,4 @@
       end
     end
   end
-
-  def sign_in_as(user)
-    user_exists_in_dfe_sign_in(user:)
-    get "/auth/dfe/callback"
-    follow_redirect!
-  end
 end

spec/support/dfe_sign_in_user_helper.rb

@@ -1,8 +1,15 @@
 module DfESignInUserHelper
   def sign_in_as(user)
     user_exists_in_dfe_sign_in(user:)
-    visit sign_in_path
-    click_on "Sign in using DfE Sign In"
+
+    case RSpec.current_example.metadata[:type]
+    when :system
+      visit sign_in_path
+      click_on "Sign in using DfE Sign In"
+    when :request
+      get "/auth/dfe/callback"
+      follow_redirect!
+    end
   end
   alias_method :given_i_sign_in_as, :sign_in_as