[ci] Add size-balanced test sequencer for better shard distribution

Jest's default test sequencer sorts alphabetically, causing large test files
(eg ReactDOMFloat-test.js at 9k lines, ReactHooksWithNoopRenderer-test.js at 4k
lines) to cluster in shard 3/5. This made shard 3/5 average 117s vs 77s for
other shards, a 52% slowdown. I'm using filesize as a rough proxy for number of tests.

This custom sequencer sorts tests by file size and distributes large files evenly across all shards
instead of clustering them together.

Author: poteto

scripts/jest/config.base.js

@@ -2,6 +2,7 @@
 
 module.exports = {
   globalSetup: require.resolve('./setupGlobal.js'),
+  testSequencer: require.resolve('./sizeBalancedSequencer.js'),
   modulePathIgnorePatterns: [
     '<rootDir>/scripts/rollup/shims/',
     '<rootDir>/scripts/bench/',

scripts/jest/sizeBalancedSequencer.js

@@ -0,0 +1,23 @@
+'use strict';
+
+const Sequencer = require('@jest/test-sequencer').default;
+const fs = require('fs');
+
+class SizeBalancedSequencer extends Sequencer {
+  shard(tests, {shardIndex, shardCount}) {
+    const shards = Array.from({length: shardCount}, () => ({tests: [], size: 0}));
+    const sorted = [...tests].sort((a, b) => fs.statSync(b.path).size - fs.statSync(a.path).size);
+
+    for (let i = 0; i < sorted.length; i++) {
+      const test = sorted[i];
+      const size = fs.statSync(test.path).size;
+      const smallest = shards.reduce((min, s) => (s.size < min.size ? s : min));
+      smallest.tests.push(test);
+      smallest.size += size;
+    }
+
+    return shards[shardIndex - 1].tests;
+  }
+}
+
+module.exports = SizeBalancedSequencer;