auto merge of #18877 : michaelsproul/rust/panic-tests, r=alexcrichton

I'm fairly sure all these tests refer to task panics. I skim read them all, but there's a small chance I renamed something too eagerly.

src/test/run-fail/unreachable-macro-panic.rs

@@ -0,0 +1,12 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// error-pattern:internal error: entered unreachable code
+fn main() { unreachable!() }