Auto merge of rust-lang#2177 - DrMeepster:global_allocator_backtrace_test, r=RalfJung

add test for backtrace with global allocator

closes rust-lang#1996

Author: bors

tests/pass/backtrace-global-alloc.rs

@@ -0,0 +1,13 @@
+// compile-flags: -Zmiri-disable-isolation
+
+#![feature(backtrace)]
+
+use std::alloc::System;
+use std::backtrace::Backtrace;
+
+#[global_allocator]
+static GLOBAL_ALLOCATOR: System = System;
+
+fn main() {
+    eprint!("{}", Backtrace::capture());
+}

tests/pass/backtrace-global-alloc.stderr

@@ -0,0 +1,28 @@
+   0: main
+ at $DIR/backtrace-global-alloc.rs:LL:CC
+   1: <fn() as std::ops::FnOnce<()>>::call_once - shim(fn())
+ at RUSTLIB/core/src/ops/function.rs:LL:CC
+   2: std::sys_common::backtrace::__rust_begin_short_backtrace
+ at RUSTLIB/std/src/sys_common/backtrace.rs:LL:CC
+   3: std::rt::lang_start::{closure#0}
+ at RUSTLIB/std/src/rt.rs:LL:CC
+   4: std::ops::function::impls::call_once
+ at RUSTLIB/core/src/ops/function.rs:LL:CC
+   5: std::panicking::r#try::do_call
+ at RUSTLIB/std/src/panicking.rs:LL:CC
+   6: std::panicking::r#try
+ at RUSTLIB/std/src/panicking.rs:LL:CC
+   7: std::panic::catch_unwind
+ at RUSTLIB/std/src/panic.rs:LL:CC
+   8: std::rt::lang_start_internal::{closure#2}
+ at RUSTLIB/std/src/rt.rs:LL:CC
+   9: std::panicking::r#try::do_call
+ at RUSTLIB/std/src/panicking.rs:LL:CC
+  10: std::panicking::r#try
+ at RUSTLIB/std/src/panicking.rs:LL:CC
+  11: std::panic::catch_unwind
+ at RUSTLIB/std/src/panic.rs:LL:CC
+  12: std::rt::lang_start_internal
+ at RUSTLIB/std/src/rt.rs:LL:CC
+  13: std::rt::lang_start
+ at RUSTLIB/std/src/rt.rs:LL:CC