Improve MaxScale startup and shutdown in NPM tests

The tests are now less violent in how they stop MaxScale. Also the logs
are now removed before a new test run is executed.

test/stop_double_maxscale.sh

@@ -1,20 +1,3 @@
 #!/bin/bash
 
-#
-# This script is run after each test block. It kills the two MaxScale processes.
-#
-
-maxscaledir=$MAXSCALE_DIR
-
-test -z "$MAXSCALE_DIR" && exit 1
-
-for ((i=0;i<200;i++))
-do
-    pkill '^maxscale$' || break
-    sleep 0.1
-done
-
-# If it wasn't dead before, now it is
-pkill -9 '^maxscale$'
-
-exit 0
+./stop_maxscale.sh