docs: add Windows test commands

[omribz156]

Summary

• Add Windows commands for building test binaries with the existing testing/build.bat helper
• Show the PowerShell form of NUMBA_CUDA_TEST_BIN_DIR
• Add language hints to the existing README command blocks

Fixes #862.

Verification

• git diff --check
• rg -n "build.bat|NUMBA_CUDA_TEST_BIN_DIR|pytest -n auto|pixi run" README.md testing pixi.toml

Full test execution was not run because this is a docs-only change and this local machine is not a CUDA-capable Windows test environment.

This was prepared with Codex assistance and manually reviewed, with the patch kept to the README test-running instructions.

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[acosmicflamingo]

Copying what I had written in original ticket so maintainers are aware while reviewing the PR:

so that build.bat script was called by ci/test_wheel.ps1 a few months ago in this #683, and it was the only thing referencing build.bat before it was removed from the codebase. Additionally, it doesn't look like it was getting as many updates as Makefile currently does. With that being said, before giving build.bat a well-needed makeover, I think we first need to know whether we are better off maintaining two distinct files, or whether we should leverage unix-tools in Windows so we can just use Makefile

TL;DR: the build.bat script hasn't been maintained for a while, so the PR in its current form isn't enough to address the issue