tests: Don't check for self-printed output in std-backtrace.rs test

The `Display` implementation for `Backtrace` used to print

    stack backtrace:

but that print was later removed. To make the existing test pass, the
print was added to the existing test. But it doesn't make sense to check
for something that the test itself does since that will not detect any
regressions in the implementation of `Backtrace`.

What the test _should_ check is that "stack backtrace:" is _not_ printed
in `Display` of `Backtrace`. So do that instead.

Author: Enselic

tests/ui/backtrace/std-backtrace.rs

@@ -13,9 +13,9 @@ use std::str;
 fn main() {
     let args: Vec<String> = env::args().collect();
     if args.len() >= 2 && args[1] == "force" {
-        println!("stack backtrace:\n{}", std::backtrace::Backtrace::force_capture());
+        println!("{}", std::backtrace::Backtrace::force_capture());
     } else if args.len() >= 2 {
-        println!("stack backtrace:\n{}", std::backtrace::Backtrace::capture());
+        println!("{}", std::backtrace::Backtrace::capture());
     } else {
         runtest(&args[0]);
         println!("test ok");
@@ -28,7 +28,9 @@ fn runtest(me: &str) {
 
     let p = Command::new(me).arg("a").env("RUST_BACKTRACE", "1").output().unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
+    // Make sure there is no header. The header to use (if any) is up the user.
+    // See https://github.com/rust-lang/rust/pull/69042.
+    assert!(!String::from_utf8_lossy(&p.stdout).contains("stack backtrace:"));
     assert!(String::from_utf8_lossy(&p.stdout).contains("backtrace::main"));
 
     let p = Command::new(me).arg("a").env("RUST_BACKTRACE", "0").output().unwrap();
@@ -46,7 +48,7 @@ fn runtest(me: &str) {
         .output()
         .unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
+    assert!(!String::from_utf8_lossy(&p.stdout).contains("stack backtrace:"));
 
     let p = Command::new(me)
         .arg("a")
@@ -64,9 +66,9 @@ fn runtest(me: &str) {
         .output()
         .unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
+    assert!(!String::from_utf8_lossy(&p.stdout).contains("stack backtrace:"));
 
     let p = Command::new(me).arg("force").output().unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
+    assert!(!String::from_utf8_lossy(&p.stdout).contains("stack backtrace:"));
 }