testing: give '-p' to mkdir

rm won't remove the containing directory if a file is removed out from under it, which means the directory will still be there but will be empty.  This is not a problem, so pass '-p' to not fail on already existing directories.

This fixes problems like:

```
rm: cannot remove ‘/home/jefftk/ngx_pagespeed/test/tmp/nginx.pid’: No such file or directory
     check mkdir /home/jefftk/ngx_pagespeed/test/tmp
mkdir: cannot create directory ‘/home/jefftk/ngx_pagespeed/test/tmp’: File exists
```