Rename feature gate test to correct filename, add stderr for uitest

Author: novafacing

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

@@ -1,3 +1,4 @@
+// edition: 2021
 // force-host
 #![crate_type = "proc-macro"]
 

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

@@ -0,0 +1,12 @@
+error[E0658]: use of unstable library feature 'c_str_literals'
+  --> $DIR/feature-gate-c_str_literals.rs:10:5
+   |
+LL |     Literal::c_string(c"a");
+   |     ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #115268 <https://github.com/rust-lang/rust/issues/105723> for more information
+   = help: add `#![feature(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`.