test: update assert messages to show expected and actual values

uses the same approach as in test-fs-readfile-pipe-large

PR-URL: #19420
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/fixtures/readfile_pipe_test.txt

@@ -0,0 +1,5 @@
+xxxx xxxx xxxx xxxx
+xxxx xxxx xxxx xxxx
+xxxx xxxx xxxx xxxx
+xxxx xxxx xxxx xxxx
+xxxx xxxx xxxx xxxx

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

@@ -30,8 +30,6 @@ if (common.isWindows || common.isAIX)
 const assert = require('assert');
 const fs = require('fs');
 
-const dataExpected = fs.readFileSync(__filename, 'utf8');
-
 if (process.argv[2] === 'child') {
   fs.readFile('/dev/stdin', function(er, data) {
     assert.ifError(er);
@@ -40,13 +38,24 @@ if (process.argv[2] === 'child') {
   return;
 }
 
+const fixtures = require('../common/fixtures');
+
+const filename = fixtures.path('readfile_pipe_test.txt');
+const dataExpected = fs.readFileSync(filename).toString();
+
 const exec = require('child_process').exec;
 const f = JSON.stringify(__filename);
 const node = JSON.stringify(process.execPath);
-const cmd = `cat ${f} | ${node} ${f} child`;
+const cmd = `cat ${filename} | ${node} ${f} child`;
 exec(cmd, function(err, stdout, stderr) {
   assert.ifError(err);
-  assert.strictEqual(stdout, dataExpected, 'it reads the file and outputs it');
-  assert.strictEqual(stderr, '', 'it does not write to stderr');
+  assert.strictEqual(
+    stdout,
+    dataExpected,
+    `expected to read: '${dataExpected}' but got: '${stdout}'`);
+  assert.strictEqual(
+    stderr,
+    '',
+    `expected not to read anything from stderr but got: '${stderr}'`);
   console.log('ok');
 });