test: update run-pass tests to not use mutable transmuting

Author: seanmonstar

src/test/auxiliary/issue_8401.rs

@@ -21,6 +21,6 @@ impl A for B {}
 fn bar<T>(_: &mut A, _: &T) {}
 
 fn foo<T>(t: &T) {
-    let b = B;
-    bar(unsafe { mem::transmute(&b as &A) }, t)
+    let mut b = B;
+    bar(&mut b as &mut A, t)
 }

src/test/run-pass/issue-2718.rs

@@ -170,7 +170,7 @@ pub mod pipes {
             unsafe {
                 if self.p != None {
                     let self_p: &mut Option<*const packet<T>> =
-                        mem::transmute(&self.p);
+                        mem::transmute(&mut self.p);
                     let p = replace(self_p, None);
                     sender_terminate(p.unwrap())
                 }
@@ -199,7 +199,7 @@ pub mod pipes {
             unsafe {
                 if self.p != None {
                     let self_p: &mut Option<*const packet<T>> =
-                        mem::transmute(&self.p);
+                        mem::transmute(&mut self.p);
                     let p = replace(self_p, None);
                     receiver_terminate(p.unwrap())
                 }