test: add test for piping large input from stdin

Check that piping a large chunk of data from `process.stdin`
into `process.stdout` does not lose any data by verifying that
the output has the same size as the input.
This is a regression test for #5927 and fails for the commits
in the range [ace1009..89abe86).
PR-URL: #5949
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: addaleax

test/parallel/test-stdin-pipe-large.js

@@ -0,0 +1,23 @@
+'use strict';
+// See https://github.com/nodejs/node/issues/5927
+
+constcommon=require('../common');
+constassert=require('assert');
+constspawn=require('child_process').spawn;
+
+if(process.argv[2]==='child'){
+process.stdin.pipe(process.stdout);
+return;
+}
+
+constchild=spawn(process.execPath,[__filename,'child'],{stdio: 'pipe'});
+
+constexpectedBytes=1024*1024;
+letreadBytes=0;
+
+child.stdin.end(Buffer(expectedBytes));
+
+child.stdout.on('data',(chunk)=>readBytes+=chunk.length);
+child.stdout.on('end',common.mustCall(()=>{
+assert.strictEqual(readBytes,expectedBytes);
+}));