re-enable system package tests (#529)

Author: dothebart

release_tester/arangodb/installers/__init__.py

@@ -80,7 +80,7 @@ def __init__(
         self.supports_rolling_upgrade = not IS_WINDOWS
         self.verbose = bc.verbose
         self.package_dir = bc.package_dir
-        self.have_system_service = False  # TODO: re-enable not self.zip_package and self.src_testing
+        self.have_system_service = not self.zip_package and self.src_testing
         self.debug_package_is_installed = False
         self.client_package_is_installed = False
         self.server_package_is_installed = False