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

Unless specified as a "raw" string, Python will try to interpret
backslashes, which means they don't get passed on correctly to the
underlying regex-engine, unless escaped manually (`\\`).

Use raw strings in `llvm/test/lit.cfg.py` to avoid a `SyntaxWarning`:
```
llvm/test/lit.cfg.py:275: SyntaxWarning: invalid escape sequence '\d'
  match = re.search("release (\d+)\.(\d+)", ptxas_out)
```

Other such warning present in 17.0.x were already fixed in
7ed0f5b.

Author: h-vetinari