test: Add progress markers to the snapshot tests

[teor2345]

Motivation

This test updates the snapshot tests with the newly added progress markers from PR #8080.

It also simplifies some related scanner code.

Close #8069

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Solution

• Write progress markers in the snapshot tests
• Simplify the latest height method
• Make marker writing more robust to future changes

Testing

This is expanding test coverage.

Review

This is a routine change.

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[arya2]

Looks good!