Split loggregator and runtime tests into independent packages

Running a single test suite can still be accomplished with (e.g.) `bin/test --focus="Loggregator:"` but can now
also be done with `bin/test smoke/logging` (which is more robust, as it is less dependent on Describe strings).

[#80791206]

Signed-off-by: John Tuley <jtuley@pivotal.io>