Extract base permission set into shared example caller

This cleans up the readability of the spec given it's used in multiple
contexts. Easier to parse when you can refer directly from the caller
you're trying to inspect.

spec/requests/api/v3/projects/available_assignees_api_spec.rb

@@ -32,6 +32,7 @@ RSpec.describe "API::V3::Projects::AvailableAssigneesAPI" do
   include API::V3::Utilities::PathHelper
 
   it_behaves_like 'available principals', :assignees do
+    let(:base_permissions) { %i[add_work_packages] }
     let(:href) { api_v3_paths.available_assignees_in_project(project.id) }
   end
 end

spec/requests/api/v3/projects/available_responsibles_api_spec.rb

@@ -32,6 +32,7 @@ RSpec.describe "API::V3::Projects::AvailableResponsiblesAPI" do
   include API::V3::Utilities::PathHelper
 
   it_behaves_like 'available principals', :responsibles do
+    let(:base_permissions) { %i[view_work_packages] }
     let(:href) { api_v3_paths.available_responsibles(project.id) }
   end
 end

spec/requests/api/v3/work_packages/available_assignees_api_spec.rb

@@ -32,6 +32,7 @@ RSpec.describe "API::V3::WorkPackages::AvailableAssigneesAPI" do
   include API::V3::Utilities::PathHelper
 
   it_behaves_like 'available principals', :assignees, work_package_scope: true do
+    let(:base_permissions) { %i[edit_work_packages view_work_packages] }
     let(:href) { api_v3_paths.available_assignees_in_work_package(work_package.id) }
   end
 end

spec/support/api/v3/shared_available_principals_examples.rb

@@ -52,13 +52,7 @@ RSpec.shared_examples_for 'available principals' do |principals, work_package_sc
     create(:placeholder_user, member_with_roles: { project => assignable_project_role })
   end
 
-  let(:permissions) do
-    if work_package_scope
-      %i[edit_work_packages view_work_packages]
-    else
-      %i[add_work_packages]
-    end
-  end
+  let(:permissions) { base_permissions }
 
   let(:assignable_permissions) { [:work_package_assigned] }
 
@@ -74,13 +68,7 @@ RSpec.shared_examples_for 'available principals' do |principals, work_package_sc
     end
 
     describe 'users' do
-      let(:permissions) do
-        if work_package_scope
-          %i[edit_work_packages view_work_packages work_package_assigned]
-        else
-          %i[add_work_packages work_package_assigned]
-        end
-      end
+      let(:permissions) { base_permissions + assignable_permissions }
 
       context 'for a single user' do
         # The current user
@@ -103,13 +91,7 @@ RSpec.shared_examples_for 'available principals' do |principals, work_package_sc
       end
 
       context 'if the user lacks the assignable permission' do
-        let(:permissions) do
-          if work_package_scope
-            %i[edit_work_packages view_work_packages]
-          else
-            %i[add_work_packages]
-          end
-        end
+        let(:permissions) { base_permissions }
 
         it_behaves_like "returns available #{principals}", 0, 0, 'User'
       end