Add #[allow(unused_mut)] for various cases that arise only in compare-mode=nll.

Author: pnkfelix

src/test/run-pass/binding/nested-matchs.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // run-pass
+#![allow(unused_mut)] // under NLL we get warning about `bar` below
 fn baz() -> ! { panic!(); }
 
 fn foo() {

src/test/run-pass/issues/issue-30371.rs

@@ -10,6 +10,7 @@
 
 // run-pass
 #![allow(unreachable_code)]
+#![allow(unused_mut)] // rust-lang/rust#54586
 #![deny(unused_variables)]
 
 fn main() {

src/test/run-pass/issues/issue-49298.rs

@@ -10,6 +10,7 @@
 
 // run-pass
 #![feature(test)]
+#![allow(unused_mut)] // under NLL we get warning about `x` below: rust-lang/rust#54499
 
 extern crate test;