Set match_path on stock items MenuItem

Without setting match_path the tab helper uses the current controller
name as criteria to activate the tab in the menu. Since the stock items
controller can be accessed for a single product or all products we need
to make sure it only matches the /stock_items path for the Stock menu
tab.

Closes #2950

backend/app/models/spree/backend_configuration.rb

@@ -115,6 +115,7 @@ def menu_items
           condition: -> { can?(:admin, Spree::StockItem) },
           label: :stock,
           url: :admin_stock_items_path,
+          match_path: '/stock_items',
           position: 3
         ),
         MenuItem.new(

backend/spec/models/spree/backend_configuration_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Spree::BackendConfiguration do
+  describe '#menu_items' do
+    subject do
+      described_class.new.menu_items
+    end
+
+    describe 'menu tab for stock items' do
+      let(:stock_menu_item) do
+        subject.detect { |item| item.label == :stock }
+      end
+
+      # Regression for https://github.com/solidusio/solidus/issues/2950
+      it 'has match_path set to /stock_items' do
+        expect(stock_menu_item.match_path).to eq('/stock_items')
+      end
+    end
+  end
+end