It appears that the arquillian-testng-standalone maven artifact is not bringing in all the required dependencies. I attempted to run some Arquiallian Drone tests without a container using TestNG and got a ClassNotFoundException on org.jboss.arquillian.core.impl.loadable.LoadableExtensionLoader.
When I included the arquillian-junt-standalone artifact, it automatically included the arquillian-core-impl-base and arquillian-test-impl-base dependencies and everything executed correctly. Looking at the two POM files it appears that the scope on those dependencies is set to "test" for the TestNG artifiact. I am guessing that this is the problem. Should I raise an issue?