compiletest: `//~ ERROR` annotations are ignored in `compile-pass` tests

Reproduction:
```rust
// compile-pass

fn main() {} //~ ERROR any misleading message
```
The test passes despite the incorrect `//~ ERROR` annotation.

Tests often have these annotations while they are WIP, but then everything is good, the test passes and is marked as `// compile-pass`, but stray `//~ ERROR` annotations may remain confusing the reader.