test: use portable EOL

The test wanted to cut huge string into 1KB strings,
for which a new line character was inserted at appropriate
places. The value is different in Windows (10, 13).
Make it portable, by making use of os.EOL semantics

Refs: #25988

PR-URL: #32104
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

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

@@ -3,6 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const path = require('path');
 const fs = require('fs');
+const os = require('os');
 const spawn = require('child_process').spawn;
 const tmpdir = require('../common/tmpdir');
 
@@ -25,8 +26,8 @@ const MB = KB * KB;
   // meanings to new line - for example, line buffering.
   // So cut the buffer into lines at some points, forcing
   // data flow to be split in the stream.
-  for (let i = 0; i < KB; i++)
-    buf[i * KB] = 10;
+  for (let i = 1; i < KB; i++)
+    buf.write(os.EOL, i * KB);
   fs.writeFileSync(file, buf.toString());
 
   cat = spawn('cat', [file]);
@@ -61,6 +62,7 @@ const MB = KB * KB;
   });
 
   wc.stdout.on('data', common.mustCall((data) => {
-    assert.strictEqual(data.toString().trim(), MB.toString());
+    // Grep always adds one extra byte at the end.
+    assert.strictEqual(data.toString().trim(), (MB + 1).toString());
   }));
 }