add UT for API about tx struct adapte to balance

[codecov[bot]]

Codecov Report

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

Comparison is base (a6c64cf) 14.43% compared to head (e7e6b61) 26.29%. Report is 1 commits behind head on feature-3.6.0.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feature-3.6.0 #195 +/- ## ================================================== + Coverage 14.43% 26.29% +11.86% ================================================== Files 6 6 Lines 1753 1753 Branches 1076 1076 ================================================== + Hits 253 461 +208 + Misses 1307 917 -390 - Partials 193 375 +182 ``` | [Flag](https://app.codecov.io/gh/FISCO-BCOS/bcos-c-sdk/pull/195/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=FISCO-BCOS) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/FISCO-BCOS/bcos-c-sdk/pull/195/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=FISCO-BCOS) | `26.29% <ø> (+11.86%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=FISCO-BCOS#carryforward-flags-in-the-pull-request-comment) to find out more.

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