Update re-enabled feature gate test, update features for functionality test

Author: novafacing

library/proc_macro/src/lib.rs

@@ -1353,7 +1353,7 @@ impl Literal {
     }
 
     /// C string literal.
-    #[stable(feature = "c_str_literals", since = "1.76.0")]
+    #[unstable(feature = "proc_macro_c_str_literals", issue = "119750")]
     pub fn c_string(string: &CStr) -> Literal {
         let quoted = format!("{:?}", string);
         assert!(quoted.starts_with('"') && quoted.ends_with('"'));

tests/ui/feature-gates/feature-gate-c_string_literals.stderr

@@ -7,5 +7,5 @@ extern crate proc_macro;
 use proc_macro::Literal;
 
 fn test() {
-    Literal::c_string(c"a"); //~ ERROR use of unstable library feature 'c_str_literals'
+    Literal::c_string(c"a"); //~ ERROR use of unstable library feature 'proc_macro_c_str_literals'
 }

tests/ui/feature-gates/feature-gate-c_string_literals.rs renamed to tests/ui/feature-gates/feature-gate-proc_macro_c_str_literals.rs

@@ -0,0 +1,12 @@
+error[E0658]: use of unstable library feature 'proc_macro_c_str_literals'
+  --> $DIR/feature-gate-proc_macro_c_str_literals.rs:10:5
+   |
+LL |     Literal::c_string(c"a");
+   |     ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #119750 <https://github.com/rust-lang/rust/issues/119750> for more information
+   = help: add `#![feature(proc_macro_c_str_literals)]` to the crate attributes to enable
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/feature-gates/feature-gate-proc_macro_c_str_literals.stderr

@@ -6,6 +6,7 @@
 #![crate_name = "proc_macro_api_tests"]
 #![feature(proc_macro_span)]
 #![feature(proc_macro_byte_character)]
+#![feature(proc_macro_c_str_literals)]
 #![deny(dead_code)] // catch if a test function is never called
 
 extern crate proc_macro;