test: increase debugging information in subprocess test

Refs: #25988 (comment)
PR-URL: #30761
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Trott

test/parallel/test-child-process-pipe-dataflow.js

@@ -37,6 +37,14 @@ const MB = KB * KB;
 cat.stdout._handle.readStart=common.mustNotCall();
 grep.stdout._handle.readStart=common.mustNotCall();
 
+// Keep an array of error codes and assert on them during process exit. This
+// is because stdio can still be open when a child process exits, and we don't
+// want to lose information about what caused the error.
+consterrors=[];
+process.on('exit',()=>{
+assert.deepStrictEqual(errors,[]);
+});
+
 [cat,grep,wc].forEach((child,index)=>{
 consterrorHandler=(thing,type)=>{
 // Don't want to assert here, as we might miss error code info.
@@ -46,7 +54,9 @@ const MB = KB * KB;
 child.stderr.on('data',(d)=>{errorHandler(d,'data');});
 child.on('error',(err)=>{errorHandler(err,'error');});
 child.on('exit',common.mustCall((code)=>{
-assert.strictEqual(code,0,`child ${index} exited with code ${code}`);
+if(code!==0){
+errors.push(`child ${index} exited with code ${code}`);
+}
 }));
 });