Move 'stirling.test' package to 'main'

Make it possible to to use 'stirling.test' in projects that depend on
'stirling-core'.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Convert HeartBtIntSpec to Scala

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
FIXME