This PR fixes test flakiness introduced in
https://github.com/zed-industries/zed/pull/52111

The flakiness was because the test was using its own RNG, rather than
using gpui's built-in support for consistently-seeded RNGs in tests. The
fix was just to adjust the test to use the same logic for computing row
and column count as was introduced in the above PR.

Release Notes:

- N/A