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

[Abe27342]

This looks reasonable as a starting point, but I think there are still interesting cases untested:

• overlapping remove (two potentially distinct subcases here--remote client 1 and remote client 2 concurrently remove the same segment, then our local client needs to interpret length of the merge block at various seq_s, and the case where it's a remote client being concurrent with the local client)
• trees that have height more than 2: PartialSequenceLengths.combine has meaningfully different logic when it's merging children leaves vs. when it's merging children that are internal nodes.

I think using code coverage of partialSequenceLengths.ts when only running this test suite is a useful metric to consider here when considering which tests to write.

If you want to work on this iteratively, I'm fine with improving the test suite in a separate PR / working incrementally. Overall state of the PR is fine, I just want to dig a bit deeper :)

[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

---

Baseline commit: 0026561

Generated by 🚫 dangerJS against 9081e16

[Abe27342]

Great, glad to have this extra coverage--I have the one minor nit on that edge case, but honestly just putting a comment there mentioning it doesn't work for that case would be ok.

[github-actions]

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