Add spec on routes regex constants

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

spec/lib/gitlab/path_regex_spec.rb

@@ -170,6 +170,11 @@ describe Gitlab::PathRegex do
       expect(described_class::TOP_LEVEL_ROUTES)
         .to contain_exactly(*top_level_words), failure_block
     end
+
+    # We ban new items in this list, see https://gitlab.com/gitlab-org/gitlab/-/issues/215362
+    it 'does not allow expansion' do
+      expect(described_class::TOP_LEVEL_ROUTES.size).to eq(41)
+    end
   end
 
   describe 'GROUP_ROUTES' do
@@ -184,6 +189,11 @@ describe Gitlab::PathRegex do
       expect(described_class::GROUP_ROUTES)
         .to contain_exactly(*paths_after_group_id), failure_block
     end
+
+    # We ban new items in this list, see https://gitlab.com/gitlab-org/gitlab/-/issues/215362
+    it 'does not allow expansion' do
+      expect(described_class::GROUP_ROUTES.size).to eq(1)
+    end
   end
 
   describe 'PROJECT_WILDCARD_ROUTES' do
@@ -195,6 +205,11 @@ describe Gitlab::PathRegex do
         end
       end
     end
+
+    # We ban new items in this list, see https://gitlab.com/gitlab-org/gitlab/-/issues/215362
+    it 'does not allow expansion' do
+      expect(described_class::PROJECT_WILDCARD_ROUTES.size).to eq(21)
+    end
   end
 
   describe '.root_namespace_route_regex' do