Change default panic handler message format.
This changes the default panic hook's message format from:
```
thread '{thread}' panicked at '{message}', {location}
```
to
```
thread '{thread}' panicked at {location}:
{message}
```
This puts the message on its own line without surrounding quotes, making it easiser to read. For example:
Before:
```
thread 'main' panicked at 'env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`', src/main.rs:4:6
```
After:
```
thread 'main' panicked at src/main.rs:4:6:
env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`
```
---
See this PR by `@nyurik,` which does that for only multi-line messages (specifically because of `assert_eq`): https://github.com/rust-lang/rust/pull/111071
This is the change that does that for *all* panic messages.
This test is supposed to ensure that full backtraces are used for ICEs.
But it doesn't actually do that -- the filtering done cannot distinguish
between a full backtrace versus a short backtrace.
So this commit changes the filtering to preserve the existence of
`__rust_{begin,end}_short_backtrace` markers, which only appear in full
backtraces. This change means the test now tests what it is supposed to
test.
Also, the existing filtering included a rule that excluded any line
starting with two spaces. This was too strong because it filtered out
some parts of the error message. (This was not a showstopper). It was
also not strong enough because it didn't work with three digit stack
frame numbers, which just started seeing after upgrading my Ubuntu
distro to 23.04 machine (this *was* a showstopper).
So the commit replaces that rule with two more precise rules, one for
lines with stack frame numbers, and one for "at ..." lines.
