[ci] Add uniqueness validation for bs request actions

Using the inbuild rspec validation test helper didn't work, because
of the association ot BsRequestType named 'type'.

=====
  1) BsRequestAction validates uniquness of type among bs requests
     Failure/Error: type_to_class_name(type_name.to_sym) || super

     ActiveRecord::SubclassNotFound:
       The single-table inheritance mechanism failed to locate the subclass: 'an arbitrary value'. This error is raised because the column 'type' is reserved for storing the class in case of inheritance. Please rename this column if you didn't intend it to be used for storing the inheritance class or overwrite BsRequestAction.inheritance_column to use another column for that information.
     # ./app/models/bs_request_action.rb:87:in `find_sti_class'
     # ./spec/models/bs_request_action_spec.rb:7:in `block (2 levels) in <top (required)>'
     # ./spec/support/logging.rb:4:in `block (2 levels) in <top (required)>'
     # ------------------
     # --- Caused by: ---
     # NameError:
     #   wrong constant name an arbitrary value
     #   ./app/models/bs_request_action.rb:87:in `find_sti_class'

src/api/spec/models/bs_request_action_spec.rb

@@ -1,5 +1,37 @@
 require 'rails_helper'
 
 RSpec.describe BsRequestAction do
+  context 'uniqueness validation of type' do
+    let(:bs_request) { create(:bs_request) }
+    let(:action_attributes) {
+      {
+        bs_request:     bs_request,
+        type:           'submit',
+        target_project: 'target_prj',
+        target_package: 'target_pkg'
+      }
+    }
+    let!(:bs_request_action) { create(:bs_request_action, action_attributes) }
+
+    it { expect(bs_request_action).to be_valid }
+
+    it 'validates uniqueness of type among bs requests, target_project and target_package' do
+      duplicated_bs_request_action = build(:bs_request_action, action_attributes)
+      expect(duplicated_bs_request_action).not_to be_valid
+      expect(duplicated_bs_request_action.errors.full_messages.to_sentence).to eq('Type has already been taken')
+    end
+
+    RSpec.shared_examples 'it skips validation for type' do |type|
+      context "type '#{type}'" do
+        it 'allows multiple bs request actions' do
+          expect(build(:bs_request_action, action_attributes.merge(type: 'add_role'))).to be_valid
+        end
+      end
+    end
+
+    it_should_behave_like 'it skips validation for type', 'add_role'
+    it_should_behave_like 'it skips validation for type', 'maintenance_incident'
+  end
+
   it { should belong_to(:bs_request).touch(true) }
 end