Move dead_code related tests to test/ui/dead-code

This helps organize the tests better.  I also renamed several of the tests to remove redundant dead-code in the path, and better match what they're testing

src/test/ui/lint/dead-code/basic.rs

@@ -0,0 +1,15 @@
+#![deny(dead_code)]
+#![allow(unreachable_code)]
+
+fn foo() { //~ ERROR function is never used
+
+    // none of these should have any dead_code exposed to the user
+    panic!();
+
+    panic!("foo");
+
+    panic!("bar {}", "baz")
+}
+
+
+fn main() {}