tests: fix test-runner for osx. Closes ##4708
tests: fix test-runner for osx. Closes ##4708

Use the more portable $$ rather than $BASHPID

(cherry picked from commit 9322f1a4d9ab301f2f6b8a404fb6012d8b50ef74)

reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin