Add PlatformConformance in tests to ensure platform specifc types follow a uniform shape

Author: iCharlesHu

Tests/SubprocessTests/PlatformConformance.swift

@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+@testable import Subprocess
+
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Bionic)
+import Bionic
+#elseif canImport(Glibc)
+import Glibc
+#elseif canImport(Musl)
+import Musl
+#elseif canImport(WinSDK)
+import WinSDK
+#endif
+
+/// This file defines protocols for platform-specific structs
+/// and adds retroactive conformances to them to ensure they all
+/// conform to a uniform shape. We opted to keep these protocols
+/// in the test target as opposed to making them public APIs
+/// because we don't directly use them in public APIs.
+
+protocol ProcessIdentifierProtocol: Sendable, Hashable, CustomStringConvertible, CustomDebugStringConvertible {
+    #if os(Windows)
+    var value: DWORD { get }
+    #else
+    var value: pid_t { get }
+    #endif
+}
+
+extension ProcessIdentifier : ProcessIdentifierProtocol {}