test: add more tests for region migration procedure

[WenyXu]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

1. Add flow test for open candidate region with retryable error
2. Add flow test for upgrade candidate retry failed
3. Add flow test for upgrade candidate with retry

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

#2700

[codecov]

Codecov Report

Merging #2895 (7a70202) into develop (5a99f09) will decrease coverage by 0.54%.
The diff coverage is 98.93%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2895      +/-   ##
===========================================
- Coverage    85.06%   84.53%   -0.54%     
===========================================
  Files          753      753              
  Lines       119059   119400     +341     
===========================================
- Hits        101281   100938     -343     
- Misses       17778    18462     +684     

[WenyXu]

@fengjiachun PTAL

[fengjiachun]

LGTM