add unit tests for the @singleton decorator
add unit tests for the @singleton decorator


Former-commit-id: 0e50a84f42bc553647db28264b9c91e884f9981f [formerly 0e50a84f42bc553647db28264b9c91e884f9981f [formerly 5a24d42d221759e708f1c258171784caab8ac8ad]]
Former-commit-id: 55013f16ee4151cfe17c8db0822fd81f06b5559f
Former-commit-id: f433e3d46ba6bff1f7220dd4e15e89f5c15fc22d
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine