test-client: fix warnings about invalid escape sequences

Summary

In Python 3.12 a backslash-character pair that is not a valid escape sequence generates a SyntaxWarning [1]:

src/tests/client/test-client.py:2161: SyntaxWarning: invalid escape sequence '\?'
nmc.pexp.expect("Do you want to provide them\? \(yes/no\) \[yes]")

Use raw strings to avoid that.

[1] https://docs.python.org/3/whatsnew/3.12.html