Clarify that 8-bit exit codes aren't a problem on macOS and Windows.

Sources/Testing/ExitTests/ExitCondition.swift

@@ -44,9 +44,9 @@ public enum ExitCondition: Sendable {
   /// | Linux | [`<stdlib.h>`](https://sourceware.org/glibc/manual/latest/html_node/Exit-Status.html), `<sysexits.h>` |
   /// | Windows | [`<stdlib.h>`](https://learn.microsoft.com/en-us/cpp/c-runtime-library/exit-success-exit-failure) |
   ///
-  /// On POSIX-like systems including macOS and Linux, only the low unsigned 8
-  /// bits (0&ndash;255) of the exit code are reliably preserved and reported to
-  /// a parent process.
+  /// On macOS and Windows, the full exit code reported by the process is
+  /// yielded to the parent process. Linux and other POSIX-like systems may only
+  /// reliably report the low unsigned 8 bits (0&ndash;255) of the exit code.
   case exitCode(_ exitCode: CInt)
 
   /// The process terminated with the given signal.

Tests/TestingTests/ExitTestTests.swift

@@ -197,6 +197,24 @@ private import _TestingInternals
       }.run(configuration: configuration)
     }
   }
+
+#if !os(Linux)
+  @Test("Exit test reports > 8 bits of the exit code")
+  func fullWidthExitCode() async {
+    // On macOS and Linux, we use waitid() which per POSIX should report the
+    // full exit code, not just the low 8 bits. This behaviour is not
+    // well-documented and while Darwin correctly reports the full value, Linux
+    // does not (at least as of this writing) and other POSIX-like systems may
+    // also have issues. This test serves as a canary when adding new platforms
+    // that we need to document the difference.
+    //
+    // Windows does not have the 8-bit exit code restriction and always reports
+    // the full CInt value back to the testing library.
+    await #expect(exitsWith: .exitCode(512)) {
+      exit(512)
+    }
+  }
+#endif
 }
 
 // MARK: - Fixtures