test: use common.js to check platform

PR-URL: https://github.com/nodejs/node/pull/12629
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

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

@@ -27,7 +27,7 @@ const path = require('path');
 
 // `fs.readFile('/')` does not fail on FreeBSD, because you can open and read
 // the directory there.
-if (process.platform === 'freebsd') {
+if (common.isFreeBSD) {
   common.skip('platform not supported.');
   return;
 }