Vault: Test update_vault_balance with reward

[aoikurokawa]

Motivation

• Fixes #88

Solution

• Add integration tests for update_vault_balance

[aoikurokawa]

@buffalu @coachchucksol It's ready for review! Thank you so much!!

[buffalu]

@buffalu @coachchucksol It's ready for review! Thank you so much!!

thank you!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.63%. Comparing base (23b35ba) to head (826f311). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #94 +/- ## ========================================== - Coverage 78.64% 78.63% -0.02% ========================================== Files 85 85 Lines 8439 8434 -5 ========================================== - Hits 6637 6632 -5 Misses 1802 1802 ```

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