added test for plugin mechanism

tests/test_unit/test_backends_plugin.py

@@ -0,0 +1,27 @@
+from postgraas_server.backends import get_backend
+from postgraas_server.backends.docker import DockerBackend
+from postgraas_server.backends.postgres_cluster import PGClusterBackend
+
+
+def test_get_backend_docker():
+    config = {"backend": {
+        "type": "docker"
+        }
+    }
+    backend_config = get_backend(config)
+    assert isinstance(backend_config, DockerBackend)
+
+
+def test_get_backend_pg_cluster():
+    config = {"backend": {
+        "type": "pg_cluster"
+        }
+    }
+    backend_config = get_backend(config)
+    assert isinstance(backend_config, PGClusterBackend)
+
+
+def test_get_backend_docker_default():
+    config = {}
+    backend_config = get_backend(config)
+    assert isinstance(backend_config, DockerBackend)