Changed unwrap_or to unwrap_or_else in some places.

The discussion seems to have resolved that this lint is a bit "noisy" in
that applying it in all places would result in a reduction in
readability.

A few of the trivial functions (like `Path::new`) are fine to leave
outside of closures.

The general rule seems to be that anything that is obviously an
allocation (`Box`, `Vec`, `vec![]`) should be in a closure, even if it
is a 0-sized allocation.

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -282,7 +282,7 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
     let mut test_runner = cx
         .test_runner
         .clone()
-        .unwrap_or(ecx.path(sp, vec![test_id, Ident::from_str_and_span(runner_name, sp)]));
+        .unwrap_or_else(|| ecx.path(sp, vec![test_id, Ident::from_str_and_span(runner_name, sp)]));
 
     test_runner.span = sp;