Revert "Remove useless feature gating test for stable feature"

This reverts commit c4b84e9.

Author: novafacing

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

@@ -0,0 +1,11 @@
+// edition: 2021
+// force-host
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::Literal;
+
+fn test() {
+    Literal::c_string(c"a"); //~ ERROR use of unstable library feature 'c_str_literals'
+}

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`.