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
+
+const common = require('../common');
+const assert = require('assert');
+const spawn = require('child_process').spawn;
+
+if (process.argv[2] === 'child') {
+  process.stdin.pipe(process.stdout);
+  return;
+}
+
+const child = spawn(process.execPath, [__filename, 'child'], { stdio: 'pipe' });
+
+const expectedBytes = 1024 * 1024;
+let readBytes = 0;
+
+child.stdin.end(Buffer(expectedBytes));
+
+child.stdout.on('data', (chunk) => readBytes += chunk.length);
+child.stdout.on('end', common.mustCall(() => {
+  assert.strictEqual(readBytes, expectedBytes);
+}));