fix(test): Fix iota-storage::key_value_tests simtests::test_multi_fetch

[bingyanglin]

Description of change

With @piotrm50 together to fix the iota-storage::key_value_tests simtests::test_multi_fetch by correcting the test_server and its get_data logic

Links to any relevant issues

Part of #3321

Type of change

• Bug fix (a non-breaking change which fixes an issue)

How the change has been tested

Passed the testing by

 ./scripts/simtest/cargo-simtest simtest \
  --color always \
  --package iota-storage --test key_value_tests -- simtests::test_multi_fetch

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes