Rollup merge of #143448 - Enselic:remote-test-client-signals, r=Mark-Simulacrum

remote-test-client: Exit code `128 + <signal-number>` instead of `3`

If the remote process is terminated by a signal, make `remote-test-client` exit with the code `128 + <signal-number>` instead of always `3`. This follows common practice among tools such as bash [^1]:

> When a command terminates on a fatal signal whose number is N, Bash uses the
> value 128+N as the exit status.

It also allows us to differentiate between `run-pass` and `run-crash` ui tests without special case code in compiletest for that when `remote-test-client` is used. See rust-lang/rust#143002 and in particular rust-lang/rust#143002 (comment).

Exiting with code `3` has been done from the start (see rust-lang/rust#39400) and seems arbitrary rather than a deliberate design decision, so changing it does not seem like an extraordinarily big deal.

### Regression testing

Note that rust-lang/rust#143002 will act as a regression test once it is rebased on this PR.

### Why a separate PR

I think it is comforting to know that CI does not break with just this change. But if my reviewer prefers, we can move this commit to be part of rust-lang/rust#143002 instead.

[^1]: https://www.gnu.org/software/bash/manual/html_node/Exit-Status.html

Author: samueltardieu