Fix ExecutionMutableStateSuite (#7026)

## What changed?
<!-- Describe what has changed in this PR -->
- Fix how branchToken is created and used in ExecutionMutableStateSuite
- More assertions
- Some variable rename

## Why?
<!-- Tell your future self why have you made these changes -->
- The test was using the same branch token for multiple different runs
and worked in a weird way (new record happens to overwrite old record in
DB).

## How did you test it?
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test? Checked in staging env? -->

## Potential risks
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change to production? -->

## Documentation
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stated in `docs/`? If significant
new behavior is added, have you described that in `docs/`? -->

## Is hotfix candidate?
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be sent to the broader community? (Yes/No) -->