[db] Add unit test for branchNode

[huof6829]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes https://github.com/iotexproject/iotex-core/issues/3201

Type of change

Please delete options that are not relevant.

• [] Bug fix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Code refactor or improvement
• [] Breaking change (fix or feature that would cause a new or changed behavior of existing functionality)
• [] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [] make test
• [] fullsync
• [] Other test (please specify)

Test Configuration:

• Firmware version:
• Hardware:
• Toolchain:
• SDK:

Checklist:

• [] My code follows the style guidelines of this project
• [] I have performed a self-review of my code
• [] I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• [] My changes generate no new warnings
• [] I have added tests that prove my fix is effective or that my feature works
• [] New and existing unit tests pass locally with my changes
• [] Any dependent changes have been merged and published in downstream modules

[codecov[bot]]

Codecov Report

Merging #3457 (16d92cf) into master (1f10a0f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3457   +/-   ##
=======================================
  Coverage   75.28%   75.28%           
=======================================
  Files         241      241           
  Lines       22258    22258           
=======================================
  Hits        16758    16758           
  Misses       4597     4597           
  Partials      903      903           

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[Liuhaai]

The trie is redesigned in the prs ( #3194, #3193 ). The test is to be added after the change is merged.