test: improve message in test-fs-readfile-pipe-large

Improve assertion message by including expected and actual values.

PR-URL: #16840
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-fs-readfile-pipe-large.js

@@ -29,8 +29,16 @@ const node = JSON.stringify(process.execPath);
 const cmd = `cat ${filename} | ${node} ${f} child`;
 exec(cmd, { maxBuffer: 1000000 }, function(err, stdout, stderr) {
   assert.ifError(err);
-  assert.strictEqual(stdout, dataExpected, 'it reads the file and outputs it');
-  assert.strictEqual(stderr, '', 'it does not write to stderr');
+  assert.strictEqual(
+    stdout,
+    dataExpected,
+    `expect it reads the file and outputs 999999 'a' but got : ${stdout}`
+  );
+  assert.strictEqual(
+    stderr,
+    '',
+    `expect that it does not write to stderr, but got : ${stderr}`
+  );
   console.log('ok');
 });