fix: fix MockInstance rebuild issue

[WenyXu]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Reuse the GreptimeStandalone KvBackendRef during the rebuilding.
The reason why I didn't rebuild the instance after dropping the KvBackendRef (and keep TestGuard), based on my experience, it's a little tricky, the FileLock used in the KvBackendRef seems not to release as soon as the KvBackendRef is dropped (maybe somewhere, still keep the KvBackendRef as well).

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

#3202

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (d75cf86) 85.85% compared to head (f5a0c1b) 85.35%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3218      +/-   ##
==========================================
- Coverage   85.85%   85.35%   -0.50%     
==========================================
  Files         844      845       +1     
  Lines      138656   138805     +149     
==========================================
- Hits       119042   118478     -564     
- Misses      19614    20327     +713