[llvm] [lit] Use raw strings for backslash escapes to fix SyntaxWarnings (PR #70907)

[via llvm-commits]

h-vetinari wrote:

@llvm/pr-subscribers-testing-tools 
The only file touched by this PR does not seem to be covered by either codeowners or the [PR labeler](https://github.com/llvm/llvm-project/blob/main/.github/new-prs-labeler.yml), and there's no team (that I could find) for lit/llvm-lit directly. Based on other PRs having the https://github.com/llvm/llvm-project/labels/llvm-lit label, I hope I'm tagging the right target audience 😅 

https://github.com/llvm/llvm-project/pull/70907