Use exec_env to set backtrace level and normalize output

Author: phansch

tests/ui/custom_ice_message.rs

@@ -1,3 +1,7 @@
+// exec-env:RUST_BACKTRACE=0
+// normalize-stderr-test: "Clippy version: .*" -> "Clippy version: foo"
+// normalize-stderr-test: "internal_lints.rs.*" -> "internal_lints.rs:1:1"
+
 #![deny(clippy::internal)]
 
 fn should_trigger_an_ice_in_clippy() {}

tests/ui/custom_ice_message.stderr

@@ -1,4 +1,4 @@
-thread 'rustc' panicked at 'Testing the ICE message', clippy_lints/src/utils/internal_lints.rs:333:13
+thread 'rustc' panicked at 'Testing the ICE message', clippy_lints/src/utils/internal_lints.rs:1:1
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
 
 error: internal compiler error: unexpected panic
@@ -7,5 +7,5 @@ note: the compiler unexpectedly panicked. this is a bug.
 
 note: we would appreciate a bug report: https://github.com/rust-lang/rust-clippy/issues/new
 
-note: Clippy version: clippy 0.0.212 (68ff8b19 2019-09-26)
+note: Clippy version: foo