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`

Author: bors

rust-version

@@ -1 +1 @@
-26373fb4baa9c5b8a7a1e2821fcfa930a85d327d
+c0127e4dbf3a9d3a67ddb1da19f8441019aab8f8

tests/run-pass/backtrace-api.rs

@@ -1,4 +1,4 @@
-// normalize-stderr-test ".*rustlib" -> "RUSTLIB"
+// normalize-stderr-test ".*/(rust|checkout)/library/" -> "RUSTLIB/"
 // normalize-stderr-test "RUSTLIB/(.*):\d+:\d+ "-> "RUSTLIB/$1:LL:COL "
 // normalize-stderr-test "::<.*>" -> ""
 

tests/run-pass/backtrace-api.stderr

@@ -2,12 +2,12 @@ $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)
+RUSTLIB/core/src/ops/function.rs:LL:COL (<fn() as std::ops::FnOnce<()>>::call_once - shim(fn()))
+RUSTLIB/std/src/sys_common/backtrace.rs:LL:COL (std::sys_common::backtrace::__rust_begin_short_backtrace)
+RUSTLIB/std/src/rt.rs:LL:COL (std::rt::lang_start::{closure#0})
+RUSTLIB/core/src/ops/function.rs:LL:COL (std::ops::function::impls::call_once)
+RUSTLIB/std/src/panicking.rs:LL:COL (std::panicking::r#try::do_call)
+RUSTLIB/std/src/panicking.rs:LL:COL (std::panicking::r#try)
+RUSTLIB/std/src/panic.rs:LL:COL (std::panic::catch_unwind)
+RUSTLIB/std/src/rt.rs:LL:COL (std::rt::lang_start_internal)
+RUSTLIB/std/src/rt.rs:LL:COL (std::rt::lang_start)