make parallel test running work locally

@tomwilkie assures me this change won't break CircleCI. We shall see.

This now works for me:

make clean
make COVERAGE=true
COVERDIR=test/coverage SLOW=true ./test/units.sh
cd test
./setup.sh
COVERAGE=true PARALLEL=true ./run_all.sh
./gen_coverage_reports.sh

test/run_all.sh

@@ -23,4 +23,4 @@ if [ -n "$CIRCLECI" -o -n "$PARALLEL" ]; then
     RUNNER_ARGS="$RUNNER_ARGS -parallel"
 fi
 
-../testing/runner/runner $RUNNER_ARGS $TESTS
+HOSTS="$HOSTS" ../testing/runner/runner $RUNNER_ARGS $TESTS