test: refactor test-fs-readfile-tostring-fail

The test uses both `assert()` and `assert.ok()`. Use just `assert.ok()`.

Remove a comment that does not appear to match the code and does not
seem to explain much beyond the bare code itself.

PR-URL: nodejs#19404
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/sequential/test-fs-readfile-tostring-fail.js

@@ -30,11 +30,10 @@ for (let i = 0; i < 201; i++) {
 
 stream.end();
 stream.on('finish', common.mustCall(function() {
-  // make sure that the toString does not throw an error
   fs.readFile(file, 'utf8', common.mustCall(function(err, buf) {
     assert.ok(err instanceof Error);
-    assert(/^(Array buffer allocation failed|"toString\(\)" failed)$/
-             .test(err.message));
+    assert.ok(/^(Array buffer allocation failed|"toString\(\)" failed)$/
+              .test(err.message));
     assert.strictEqual(buf, undefined);
   }));
 }));