[1.1] Randomized test failure on CI

From https://ci.swift.org/view/Swift%20Packages%20-%202/view/swift-collections/view/release_1.1/job/nightly_main_macos/job/swift-collections/job/release%252F1.1/3/console:

```
Test Case '-[RopeModuleTests.TestBigString test_insert_string]' started.
Global seed: 466198225361778071
Stride: 1
_RopeModule/BigString+Chunk+Splitting.swift:48: Assertion failed
```

This job is using a nightly snapshot, but the rope tests are randomized, and this could easily indicate a subtle logic error that affects shipping toolchains, too. The seed number above will let us reproduce the failure to analyze it.

Start with RopeModuleTests.TestBigString.test_insert_string and reproduce the failure using seed 466198225361778071 from the nightly CI log. Inspect _RopeModule/BigString+Chunk+Splitting.swift at line 48 and trace the assertion failure. Done means the cause is addressed and the randomized test passes reliably.

Written by the indexing model from the issue text.