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`. Fully remove the
checks.

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 +28,6 @@ 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"));
     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 +45,6 @@ fn runtest(me: &str) {
         .output()
         .unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
 
     let p = Command::new(me)
         .arg("a")
@@ -64,9 +62,7 @@ fn runtest(me: &str) {
         .output()
         .unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
 
     let p = Command::new(me).arg("force").output().unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
 }