Don't set RUST_BACKTRACE for tests

It is slow but it can be useful and can be set/unset explicitly before
running tests if needed. There is no backtrace by default anyway.

tests/compile-test.rs

@@ -21,7 +21,6 @@ fn run_mode(dir: &'static str, mode: &'static str) {
 
 fn prepare_env() {
     set_var("CLIPPY_DISABLE_WIKI_LINKS", "true");
-    set_var("RUST_BACKTRACE", "0"); // these are riddicously slow right now
 }
 
 #[test]