Renamed ui/dead-code-ret to ui/unreachable-code-ret

This test was actually about the unreachable_code flag, not dead_code,
so I renamed it for clarity (to prepare for the next commit, where I
plan to move a bunch of the dead_code tests to a single folder)

Author: Quantumplation

src/test/ui/dead-code-ret.rs renamed to src/test/ui/unreachable-code-ret.rs

@@ -1,13 +1,13 @@
 error: unreachable statement
-  --> $DIR/dead-code-ret.rs:7:5
+  --> $DIR/unreachable-code-ret.rs:7:5
    |
 LL |     return;
    |     ------ any code following this expression is unreachable
 LL |     println!("Paul is dead");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable statement
    |
 note: lint level defined here
-  --> $DIR/dead-code-ret.rs:3:9
+  --> $DIR/unreachable-code-ret.rs:3:9
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^