test: improve test coverage for os package

Author: juanarbol

test/parallel/test-os-process-priority.js

@@ -109,6 +109,16 @@ for (let i = PRIORITY_HIGHEST; i <= PRIORITY_LOW; i++) {
   checkPriority(process.pid, i);
 }
 
+{
+  // From libuv docs
+  // "The priority value range is between -20 (high priority) and 19 (low priority)"
+  assert.throws(() => { os.getPriority(-21); }, {
+    code: 'ERR_SYSTEM_ERROR',
+    message: 'A system error occurred: uv_os_getpriority returned ESRCH (no such process)',
+    name: 'SystemError'
+  });
+}
+
 
 function checkPriority(pid, expected) {
   const priority = os.getPriority(pid);