Add test code for subscribe

[pott-cho]

Close: #267
Added new integration tests:

• sampling_policy_flow_with_fake_giganto_server: Validates the basic flow of policy registration and data reception.
• sampling_policy_notify_flow_with_delete: Verifies stream termination and local file cleanup upon policy deletion.
• sampling_policy_multiple_streams: Ensures individual streams operate correctly when multiple policies are active simultaneously.

Enhanced test readability:

• timeseries_with_conn: Now validates that multiple Conn events within the same 15-minute slot are correctly aggregated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.58%. Comparing base (9f1fb86) to head (0342f07).

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #283       +/-   ##
===========================================
+ Coverage   10.96%   48.58%   +37.62%     
===========================================
  Files           7        7               
  Lines         529      531        +2     
===========================================
+ Hits           58      258      +200     
+ Misses        471      273      -198     

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