test: ensure that test priority is not higher than current priority

PR-URL: #55739
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

test/parallel/test-os.js

@@ -83,11 +83,12 @@ assert.ok(hostname.length > 0);
 
 // IBMi process priority is different.
 if (!common.isIBMi) {
-  const DUMMY_PRIORITY = 10;
-  os.setPriority(DUMMY_PRIORITY);
+  const { PRIORITY_BELOW_NORMAL, PRIORITY_LOW } = os.constants.priority;
+  const LOWER_PRIORITY = os.getPriority() > PRIORITY_BELOW_NORMAL ? PRIORITY_BELOW_NORMAL : PRIORITY_LOW;
+  os.setPriority(LOWER_PRIORITY);
   const priority = os.getPriority();
   is.number(priority);
-  assert.strictEqual(priority, DUMMY_PRIORITY);
+  assert.strictEqual(priority, LOWER_PRIORITY);
 }
 
 // On IBMi, os.uptime() returns 'undefined'