[common/persistence] Unit tests for SerializeWorkflowSnapshot

[3vilhamster]

What changed?
Covered SerializeWorkflowSnapshot function

Why?
Improving code coverage of ExecutionManager.

How did you test it?
Unit tests.

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.09%. Comparing base (02c7efb) to head (f7303b1).
Report is 7 commits behind head on master.

❗ Current head f7303b1 differs from pull request most recent head 7bcae2a

Please upload reports for the commit 7bcae2a to get more accurate results.

Additional details and impacted files

see 6 files with indirect coverage changes

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