Auto merge of #1566 - RalfJung:backtrace, r=RalfJung

normalize backtrace stderr even more

The test previously failed on rustc CI with this diff:
```
 $DIR/backtrace-api.rs:21:59 (func_c)
 $DIR/backtrace-api.rs:20:53 (func_b)
 $DIR/backtrace-api.rs:19:50 (func_a)
 $DIR/backtrace-api.rs:25:18 (main)
-RUSTLIB/src/rust/library/core/src/ops/function.rs:LL:COL (<fn() as std::ops::FnOnce<()>>::call_once - shim(fn()))
-RUSTLIB/src/rust/library/std/src/sys_common/backtrace.rs:LL:COL (std::sys_common::backtrace::__rust_begin_short_backtrace)
-RUSTLIB/src/rust/library/std/src/rt.rs:LL:COL (std::rt::lang_start::{closure#0})
-RUSTLIB/src/rust/library/core/src/ops/function.rs:LL:COL (std::ops::function::impls::call_once)
-RUSTLIB/src/rust/library/std/src/panicking.rs:LL:COL (std::panicking::r#try::do_call)
-RUSTLIB/src/rust/library/std/src/panicking.rs:LL:COL (std::panicking::r#try)
-RUSTLIB/src/rust/library/std/src/panic.rs:LL:COL (std::panic::catch_unwind)
-RUSTLIB/src/rust/library/std/src/rt.rs:LL:COL (std::rt::lang_start_internal)
-RUSTLIB/src/rust/library/std/src/rt.rs:LL:COL (std::rt::lang_start)
+/checkout/library/core/src/ops/function.rs:227:5 (<fn() as std::ops::FnOnce<()>>::call_once - shim(fn()))
+/checkout/library/std/src/sys_common/backtrace.rs:137:18 (std::sys_common::backtrace::__rust_begin_short_backtrace)
+/checkout/library/std/src/rt.rs:66:18 (std::rt::lang_start::{closure#0})
+/checkout/library/core/src/ops/function.rs:259:13 (std::ops::function::impls::call_once)
+/checkout/library/std/src/panicking.rs:381:40 (std::panicking::r#try::do_call)
+/checkout/library/std/src/panicking.rs:345:19 (std::panicking::r#try)
+/checkout/library/std/src/panic.rs:382:14 (std::panic::catch_unwind)
+/checkout/library/std/src/rt.rs:51:25 (std::rt::lang_start_internal)
+/checkout/library/std/src/rt.rs:65:5 (std::rt::lang_start)
```
Cc `@Aaron1011`