test: make child backpressure event ordering deterministic

tests/corpus/2894-child-stdin-backpressure.ts

@@ -8,8 +8,11 @@ const output = child.stdout;
 if (input === null || output === null) throw new Error("missing pipe");
 
 let count = "";
+let outputEnded = false;
+let exited = false;
+let exitCode: number | null = null;
 output.on("data", (chunk) => { count += chunk.toString(); });
-output.on("end", () => { console.log("count", count.trim()); });
+output.on("end", () => { outputEnded = true; console.log("count", count.trim()); });
 input.on("drain", () => {
   console.log("drain", input.writable);
   input.end();
@@ -18,4 +21,9 @@ input.on("finish", () => { console.log("finish", input.writable); });
 input.on("error", (err) => { console.log("error", err.message); });
 console.log("boundary", input.write(Buffer.alloc(64 * 1024 + 1, 66)));
 console.log("write", input.write(Buffer.alloc(1024 * 1024, 65)));
-child.on("exit", (code) => { console.log("exit", code); });
+child.on("exit", (code) => { exited = true; exitCode = code; });
+// Exit and stdout EOF can arrive in either order. Close follows both.
+child.on("close", () => {
+  if (!exited || !outputEnded) throw new Error("close before exit or stdout end");
+  console.log("exit", exitCode);
+});