Define primitive lock type for WASI

WASI without `_runtime(multithreaded)` does not support locks. This
commit defines the underlying lock type as `Void` in this case.

`ManagedBuffer<T, Void>` just wastes a word of storage space, so it's
not optimal but it's good at code sharing across platforms.

Author: kateinoigakukun

Sources/Testing/Support/Locked.swift

@@ -42,6 +42,7 @@ struct Locked<T>: RawRepresentable, Sendable where T: Sendable {
   private typealias _Lock = SRWLOCK
 #elseif os(WASI)
   // No locks on WASI without multithreaded runtime.
+  private typealias _Lock = Void
 #else
 #warning("Platform-specific implementation missing: locking unavailable")
   private typealias _Lock = Void