split up unit-tests and use 4 containers (they're free)

tests/ci/parallel_runner.sh

@@ -2,16 +2,21 @@
 
 case "$CIRCLE_NODE_INDEX" in
   0)
-    echo "=====> make unit-tests"
-    sudo -E make -e unit-tests
+    echo "=====> make unit-tests (1/2)"
+    sudo -E UNIT_TEST_BATCH=1 make -e unit-tests
     ;;
 
   1)
+    echo "=====> make unit-tests (2/2)"
+    sudo -E UNIT_TEST_BATCH=2 make -e unit-tests
+    ;;
+
+  2)
     echo "=====> make deploy-tests (buildstep release)"
     sudo -E make -e deploy-tests
     ;;
 
-  2)
+  3)
     echo "=====> make deploy-tests (buildstep master)"
     docker rmi -f progrium/buildstep && \
     sudo -E BUILD_STACK=true make -e stack && \