Add unit test target to pod sample.

Sample/Podfile

@@ -2,6 +2,6 @@ platform :ios, '9.0'
 use_frameworks!
 
 target 'SampleForPod' do
-  pod 'GoogleSignIn', :path => '../'
+  pod 'GoogleSignIn', :path => '../', :testspecs => ['unit']
   project 'SignInSampleForPod.xcodeproj'
 end