Add parallel testing

Add parallel testing

This saves us a good deal of time, even allowing for process startup and
general overhead. Before:

  real    6m39.103s
  user    1m9.561s
  sys     0m2.705s

After:

  real    4m5.267s
  user    1m24.527s
  sys     0m3.828s

This requires N databases, where N is the N of processors on the host.
We need to update the docker configuration for this.

Signed-off-by: Stephen Finucane <stephen@that.guru>