Fix broken incremental test

This test does not actually emit any warnings, since
`#![allow(warnings)]` was specified. `compiletest` was erroneously
ignoring `//~` tests and looking only for `//[X]~` ones. As a result of
the changes in the previous commit, we now look for `//~` comments in
incremental tests and expect them to appear in *all* revisions.

Author: ecstatic-morse

src/test/incremental/warnings-reemitted.rs

@@ -2,9 +2,8 @@
 // compile-flags: -Coverflow-checks=on
 // build-pass (FIXME(62277): could be check-pass?)
 
-#![allow(warnings)]
 #![warn(const_err)]
 
 fn main() {
-    255u8 + 1; //~ WARNING this expression will panic at run-time
+    let _ = 255u8 + 1; //~ WARNING attempt to add with overflow
 }