Fix downloader test tear down order.

KingfisherTests/ImageDownloaderTests.swift

@@ -50,9 +50,9 @@ class ImageDownloaderTests: XCTestCase {
     
     override func tearDown() {
         // Put teardown code here. This method is called after the invocation of each test method in the class.
-        super.tearDown()
         LSNocilla.sharedInstance().clearStubs()
         downloader = nil
+        super.tearDown()
     }
     
     func testDownloadAnImage() {