[mypyc] Merge test cases in run-singledispatch.test - #22248

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, gabrielfalcao/pyenv-action@5668dcb49bd9d228bba29dd5e1fbb6e0aae55a71. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/