add 'const' to notification params type
rename notification 'const void*' parameter to 'argument'
remove unused #include from NotificationCenterTest.cpp
add newline at the end of NotificationCenterTest.cpp
fix some typos in ZNotificationCenter's tests
rename ZServiceManager::Resolve to GetService
add success status as return value of ZServiceManager::Register
add check for nullptr instance for ZServiceManager::Register
add bunch of tests for ZServiceManager
add tests for positive success status for ZNotificationCenter's methods