Add to delete the docker images of tests in local

[joanhey]

Now each framework than is test it in local, we finish with a new image.
Some devs need to test a lot of frameworks, and finish with a lot of docker images than use space for nothing, as any new ./tfb ... create the image again.

Please add:
./tfb --prune images

Yes we can use the docker prune, but we have another images than are not from this bench.

We can do manually docker rmi $(docker images -q 'techempower/tfb.test.*') but it's easier for all to have a command to delete all the test images of the bench.

Thank you.

[NateBrady23]

Closing as part of bulk cleanup.