Force RUST_BACKTRACE=0

This avoids test diffs when RUST_BACKTRACE=1 is set in in the shell
config.

Author: bjorn3

tests/integration.rs

@@ -32,6 +32,11 @@ fn main() -> Result<()> {
         .envs
         .push(("BLESS".into(), (!args.check).then(|| String::new().into())));
 
+    config
+        .program
+        .envs
+        .push(("RUST_BACKTRACE".into(), Some("0".into())));
+
     config.stdout_filter("in ([0-9]m )?[0-9\\.]+s", "");
     config.stdout_filter(r#""--out-dir"(,)? "[^"]+""#, r#""--out-dir"$1 "$$TMP"#);
     config.filter("\\.exe", b"");

tests/integrations/basic-fail/Cargo.stdout

@@ -373,7 +373,7 @@ error: internal compiler error: no errors reported for args
 
 thread 'rustc' panicked at compiler/rustc_errors/src/lib.rs:
 aborting due to `-Z treat-err-as-bug=1`
-stack backtrace:
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 
 error: the compiler unexpectedly panicked. this is a bug.