Description

Fixes: #8244

In current implementation, test files are distributed with Math.ceil(files.length / count) in {index}/{count} shard setting.
This causes #8244 because this ceil based distribution logic can consume all test files by shards without the ending shard.
In addition to this, I believe this logic causes a bit lower performance in an edge-case like:

• test files: 10
• --shard index/3

In the above case, test files will be distributed as [3, 3, 3, 1] but I think it is more ideal to be distributed as [3, 3, 2, 2] in the point of parallelism.

This PR fixes this issue by introducing distribution logic based on Math.floor(files.length / count) instead of Math.ceil

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