Fixes test case TestNSFileManger.test_fileAttributes

Check only for the existence of file owner's account (user) identifier instead of comparing its numeric value because
- user identifier of `0` are assigned to the superuser in linux
- negative value are valid account identifier. e.g. `nobody` is `-2` (See https://en.wikipedia.org/wiki/User_identifier)
- There's an assumption made here that account identifier are numeric which is reasonable for most system but there's a slim chance that this might not be the case.

Thanks to @lxcid

Author: bolshedvorsky

TestFoundation/TestNSFileManager.swift

@@ -121,7 +121,7 @@ class TestNSFileManger : XCTestCase {
             XCTAssertEqual(fileType!, NSFileTypeRegular)
 
             let fileOwnerAccountID = attrs[NSFileOwnerAccountID] as? NSNumber
-            XCTAssertNotEqual(fileOwnerAccountID!.longLongValue, 0)
+            XCTAssertNotNil(fileOwnerAccountID)
 
         } catch let err {
             XCTFail("\(err)")