Add Rake tasks openfoodnetwork:specs:engines:NAME:rspec

The task name follows the directory name under engines/. For example,
for the Web engine in engines/web, run:

    bundle exec rake openfoodnetwork:specs:engines:web:rspec

lib/tasks/specs.rake

@@ -0,0 +1,30 @@
+namespace :openfoodnetwork do
+  namespace :specs do
+    namespace :engines do
+      def detect_engine_paths
+        Pathname("engines/").children.select(&:directory?)
+      end
+
+      def engine_name_for_engine(engine_path)
+        engine_path.basename.to_path
+      end
+
+      def execute_rspec_for_engine(engine_path)
+        system "cd #{engine_path.expand_path} && bundle exec rspec"
+      end
+
+      engine_paths = detect_engine_paths
+
+      engine_paths.each do |engine_path|
+        engine_name = engine_name_for_engine(engine_path)
+
+        namespace engine_name do
+          task :rspec do
+            success = execute_rspec_for_engine(engine_path)
+            abort "Failure when running tests for engine \"#{engine_name}\"" unless success
+          end
+        end
+      end
+    end
+  end
+end