Restore missing "skip windows" logic in test (#18724)

There is a comment to skip on windows, but we forgot to actually skip. Skip!

Author: donoghuc

logstash-core/src/test/java/org/logstash/settings/WritableDirectorySettingTest.java

@@ -213,6 +213,7 @@ public void whenDirectoryMissingAndParentNotWritableThenValidateFails() throws I
     @Test
     public void whenDirectoryMissingAndParentNotWritableThenValidateValueFails() throws IOException {
         // Skip on Windows where chmod doesn't work the same way
+        assumeTrue(isNotWindowsOS());
         assertFalse("Can't run as root since root can write anywhere (no real read-only files)", isRoot());
 
         Path parentDir = tempFolder.newFolder("parent").toPath();