CR: renaming var to be more general

// FREEBIE

Example/TSKitiOSTestApp/Podfile

@@ -21,7 +21,7 @@ post_install do |installer|
           existing_definitions = "$(inheritied)"
         end
 
-        config.build_settings['GCC_PREPROCESSOR_DEFINITIONS'] = "#{existing_definitions} SSK_NO_ENFORCE_SINGLETONS=1"
+        config.build_settings['GCC_PREPROCESSOR_DEFINITIONS'] = "#{existing_definitions} SSK_BUILDING_FOR_TESTS=1"
       end
     end
   end

src/Util/Asserts.h

@@ -71,7 +71,7 @@
 // 1. Use OWSSingletonAssertFlag() outside the class definition.
 // 2. Use OWSSingletonAssertInit() in each initializer.
 
-#ifndef SSK_NO_ENFORCE_SINGLETONS
+#ifndef SSK_BUILDING_FOR_TESTS
 #ifdef DEBUG
 
 #define ENFORCE_SINGLETONS