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.

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

Author: Enselic

src/tools/remote-test-client/src/main.rs

@@ -335,7 +335,9 @@ fn run(support_lib_count: usize, exe: String, all_args: Vec<String>) {
         std::process::exit(code);
     } else {
         println!("died due to signal {}", code);
-        std::process::exit(3);
+        // Behave like bash and other tools and exit with 128 + the signal
+        // number. That way we can avoid special case code in other places.
+        std::process::exit(128 + code);
     }
 }