test: add os setPriority, getPriority test coverage

PR-URL: nodejs#38771
Reviewed-By: Darshan Sen <raisinten@gmail.com>

Author: waelsy123

test/parallel/test-os.js

@@ -81,6 +81,12 @@ const hostname = os.hostname();
 is.string(hostname);
 assert.ok(hostname.length > 0);
 
+const DUMMY_PRIORITY = 10;
+os.setPriority(DUMMY_PRIORITY);
+const priority = os.getPriority();
+is.number(priority);
+assert.strictEqual(priority, DUMMY_PRIORITY);
+
 // On IBMi, os.uptime() returns 'undefined'
 if (!common.isIBMi) {
   const uptime = os.uptime();