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 since removed. See https://github.com/rust-lang/backtrace-rs/pull/286 and https://github.com/rust-lang/rust/pull/69042. To make the existing test pass, the print was added to the test instead. 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. This is one small steps towards resolving https://github.com/rust-lang/rust/issues/71706. The next steps after this step involves extending and hardening that test further.