test: add "mustCall" to test-fs-readfile-unlink

Add common.mustCall to test-fs-readfile-unlink

PR-URL: #27453
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

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

@@ -20,7 +20,7 @@
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 'use strict';
-require('../common');
+const common = require('../common');
 
 // Test that unlink succeeds immediately after readFile completes.
 
@@ -37,12 +37,12 @@ tmpdir.refresh();
 
 fs.writeFileSync(fileName, buf);
 
-fs.readFile(fileName, function(err, data) {
+fs.readFile(fileName, common.mustCall((err, data) => {
   assert.ifError(err);
   assert.strictEqual(data.length, buf.length);
   assert.strictEqual(buf[0], 42);
 
   // Unlink should not throw. This is part of the test. It used to throw on
   // Windows due to a bug.
   fs.unlinkSync(fileName);
-});
+}));