Simplify rawstrings in tests (#10180)

Inspired by
https://rust-lang.github.io/rust-clippy/master/index.html#/needless_raw_string_hashes

Ran `cargo +stable clippy --workspace --all-targets`

Fixed manually as I ran into a false positive along the lines of:
https://github.com/rust-lang/rust-clippy/issues/11068

Also collapse one set of single line tests.

Work for #8670

crates/nu-command/tests/format_conversions/csv.rs

@@ -254,12 +254,12 @@ fn from_csv_text_with_custom_escapes_to_table() {
 
         let actual = nu!(
             cwd: dirs.test(), pipeline(
-            r#"
+            r"
                 open los_tres_caballeros.txt
                 | from csv --escape '\'
                 | first
                 | get first_name
-            "#
+            "
         ));
 
         assert_eq!(actual.out, "And\"rés");