=pro #13928 Run multi-jvm tests in samples in normal test target

I'm fine with removing this == including multi-jvm tests in all test runs (without setting this env param (`  -Dakka.test.tags.include=long-running`). 

Since now removed, the note from `building-akka.rst` should also be removed, see here: https://github.com/akka/akka/blob/master/akka-docs/rst/dev/building-akka.rst#long-running-and-time-sensitive-tests