Disabled didReceiveChallenge tests on iOS 8.x due to respondsToSelector bug.

Tests/SessionDelegateTests.swift

@@ -34,10 +34,6 @@ class SessionDelegateTestCase: BaseTestCase {
         manager = Alamofire.Manager(configuration: NSURLSessionConfiguration.ephemeralSessionConfiguration())
     }
 
-    override func tearDown() {
-        super.tearDown()
-    }
-
     // MARK: - Tests - Session Invalidation
 
     func testThatSessionDidBecomeInvalidWithErrorClosureIsCalledWhenSet() {
@@ -66,6 +62,7 @@ class SessionDelegateTestCase: BaseTestCase {
     // MARK: - Tests - Session Challenges
 
     func testThatSessionDidReceiveChallengeClosureIsCalledWhenSet() {
+        if #available(iOS 9.0, *) {
             // Given
             let expectation = expectationWithDescription("Override closure should be called")
 
@@ -88,9 +85,16 @@ class SessionDelegateTestCase: BaseTestCase {
             // Then
             XCTAssertTrue(overrideClosureCalled)
             XCTAssertEqual(response?.statusCode, 200)
+        } else {
+            // This test MUST be disabled on iOS 8.x because `respondsToSelector` is not being called for the
+            // `URLSession:didReceiveChallenge:completionHandler:` selector when more than one test here is run
+            // at a time. Whether we flush the URL session of wipe all the shared credentials, the behavior is
+            // still the same. Until we find a better solution, we'll need to disable this test on iOS 8.x.
+        }
     }
 
     func testThatSessionDidReceiveChallengeWithCompletionClosureIsCalledWhenSet() {
+        if #available(iOS 9.0, *) {
             // Given
             let expectation = expectationWithDescription("Override closure should be called")
 
@@ -113,6 +117,12 @@ class SessionDelegateTestCase: BaseTestCase {
             // Then
             XCTAssertTrue(overrideClosureCalled)
             XCTAssertEqual(response?.statusCode, 200)
+        } else {
+            // This test MUST be disabled on iOS 8.x because `respondsToSelector` is not being called for the
+            // `URLSession:didReceiveChallenge:completionHandler:` selector when more than one test here is run
+            // at a time. Whether we flush the URL session of wipe all the shared credentials, the behavior is
+            // still the same. Until we find a better solution, we'll need to disable this test on iOS 8.x.
+        }
     }
 
     // MARK: - Tests - Redirects