feat(merge-tree): explicitly test partial lengths

[connorskees]

Description

More explicitly tests merge-tree partialLength logic. Additionally, moves the verify methods to be free functions.

ado 1434

[msfluid-bot]

⯆ @fluid-example/bundle-size-tests: -231 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	392.36 KB	392.28 KB	⯆ -77 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	191.92 KB	191.92 KB	■ No change
loader.js	151.12 KB	151.12 KB	■ No change
map.js	42.63 KB	42.63 KB	■ No change
matrix.js	131.7 KB	131.63 KB	⯆ -77 Bytes
odspDriver.js	150.23 KB	150.23 KB	■ No change
odspPrefetchSnapshot.js	38.39 KB	38.39 KB	■ No change
sharedString.js	152.34 KB	152.26 KB	⯆ -77 Bytes
Total Size	1.25 MB	1.25 MB	⯆ -231 Bytes

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Baseline commit: 0026561f657561ac9b02bc05e24cedec17c2d6b0

Generated by :no_entry_sign: dangerJS against 9081e169679207e1965533bc93b8103c7f85405c

[github-actions[bot]]

This commit is queued for merging with the next branch! Please ignore this PR for now. Contact @microsoft/fluid-cr-infra for help.