Parallelize make command for integration test runner

Also adds a new var, used by the prepare_tests role, to prevent it from
deleting the temp test directory at the start of each play to avoid any
potential race conditions

test/integration/roles/prepare_tests/tasks/main.yml

@@ -22,6 +22,7 @@
   always_run: True
   tags:
     - prepare
+  when: clean_working_dir|default("yes")|bool
 
 - name: create the test directory
   file: name={{output_dir}} state=directory