Fix more failing tests

Author: varkor

src/test/mir-opt/const_prop/ref_deref.rs

@@ -1,3 +1,5 @@
+#![allow(unused_must_use)]
+
 fn main() {
     *(&4);
 }

src/test/run-fail/binop-fail-3.rs

@@ -5,5 +5,5 @@ fn foo() -> ! {
 
 #[allow(resolve_trait_on_defaulted_unit)]
 fn main() {
-    foo() == foo(); // these types wind up being defaulted to ()
+    let _ = foo() == foo(); // these types wind up being defaulted to ()
 }

src/test/run-fail/binop-panic.rs

@@ -4,5 +4,5 @@ fn my_err(s: String) -> ! {
     panic!("quux");
 }
 fn main() {
-    3_usize == my_err("bye".to_string());
+    let _ = 3_usize == my_err("bye".to_string());
 }

src/test/run-fail/generator-resume-after-panic.rs

@@ -15,7 +15,7 @@ fn main() {
         panic!();
         yield;
     };
-    panic::catch_unwind(panic::AssertUnwindSafe(|| {
+    let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| {
         let x = Pin::new(&mut g).resume();
     }));
     Pin::new(&mut g).resume();

src/test/run-fail/issue-28934.rs

@@ -19,5 +19,5 @@ impl<'i, 't> Parser<'i, 't> {
 
 fn main() {
     let x = 0u8;
-    Parser(&x, &x).parse_nested_block(|input| input.expect_exhausted()).unwrap();
+    let _ = Parser(&x, &x).parse_nested_block(|input| input.expect_exhausted()).unwrap();
 }

src/test/run-fail/panic-set-unset-handler.rs

@@ -6,6 +6,6 @@ fn main() {
     panic::set_hook(Box::new(|i| {
         eprint!("greetings from the panic handler");
     }));
-    panic::take_hook();
+    let _ = panic::take_hook();
     panic!("foobar");
 }

src/test/run-fail/panic-take-handler-nop.rs

@@ -3,6 +3,6 @@
 use std::panic;
 
 fn main() {
-    panic::take_hook();
+    let _ = panic::take_hook();
     panic!("foobar");
 }