[TLC][checkpoint] Add unit test for StatefulComponentCheckpointAgent
Summary: as titiled
Test Plan:
tsloop --mode-dev-nosan aiplatform/modelstore/client/tests/:stateful_component_checkpoint_agent_test -- --focus --fail-fast
buck build mode/dev-nosan //aiplatform/modelstore/client/tests/:stateful_component_checkpoint_agent_test
./buck-out/gen///aiplatform/modelstore/client/tests//stateful_component_checkpoint_agent_test#binary.par --focus --fail-fast
Reviewed By: xunnanxu
Differential Revision: D34284271
fbshipit-source-id: 58f84c69782a7bdb30bed0a2420c74e7b7487bb9
(cherry picked from commit a1037118f4ad35335389bee5f8f75332090611ab)
