process::unix: Test wait status formatting

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/sys/unix/process/process_unix.rs

@@ -542,3 +542,7 @@ impl fmt::Display for ExitStatus {
         }
     }
 }
+
+#[cfg(test)]
+#[path = "process_unix/tests.rs"]
+mod tests;

library/std/src/sys/unix/process/process_unix/tests.rs

@@ -0,0 +1,22 @@
+#[test]
+fn exitstatus_display_tests() {
+    // In practice this is the same on every Unix.
+    // If some weird platform turns out to be different, and this test fails, use #[cfg].
+    use crate::os::unix::process::ExitStatusExt;
+    use crate::process::ExitStatus;
+
+    let t = |v, s| assert_eq!(s, format!("{}", <ExitStatus as ExitStatusExt>::from_raw(v)));
+
+    t(0x0000f, "signal: 15");
+    t(0x0008b, "signal: 11 (core dumped)");
+    t(0x00000, "exit code: 0");
+    t(0x0ff00, "exit code: 255");
+    t(0x0137f, "stopped (not terminated) by signal: 19");
+    t(0x0ffff, "continued (WIFCONTINUED)");
+
+    // Testing "unrecognised wait status" is hard because the wait.h macros typically
+    // assume that the value came from wait and isn't mad.  With the glibc I have here
+    // this works:
+    #[cfg(target_env = "gnu")]
+    t(0x000ff, "unrecognised wait status: 255 0xff");
+}