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/run_npm_test.sh

@@ -18,6 +18,8 @@ testdir=$3
 
 maxscaledir=$PWD/maxscale_test/
 
+rm -f $maxscaledir/maxscale{,1,2}.output $maxscaledir/{,secondary/}log/maxscale/maxscale.log
+
 # Create the test directories
 mkdir -p $maxscaledir $testdir