tests/sys/shell: escape backslash in test input

[Old-Ding]

Summary

• escape the backslash in the shell escaping test input so Python no longer treats \ as an invalid escape sequence
• remove the now-unneeded # noqa: W605 suppression

Verification

• python compile check with SyntaxWarning captured for tests/sys/shell/tests/01-run.py reports TOTAL 0
• git diff --check
• git ls-files --eol tests/sys/shell/tests/01-run.py

I also checked for open duplicate PRs/issues mentioning invalid escape, W605, and this test file, and found no matches.

I did not run the full shell test because it depends on a RIOT board/terminal test environment that is not available locally.

Declaration of AI-Tools / LLMs usage:

AI-Tools / LLMs that were used are:

• OpenAI Codex for code generation and local static review, with user-directed agent workflow.

[github-actions]

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[AnnsAnns]

Before we go any further, I want to strongly emphasize just how important it is to get a proper, working development environment set up — this really is the foundation upon which everything else depends. That said, I'd also love to learn more about the backstory here — specifically, how did you first stumble upon this issue? Was this something that came up organically while you were actively using this module for a particular purpose or use case? Any detail you can share, however small it may seem, would be genuinely useful. On top of that, whenever you get the chance, would you be able to send over a photo showing your test setup in its current state — and, alongside that, paste in the complete, word-for-word log output from your test run? Having both the visual and the raw text will make it far easier to fully understand — and ultimately resolve — the situation.