plotter: testing is not idempotent with respect to failure

Created by: kortschak

Invoking go test on github.com/gonum/plotter is not idempotent if there is a failure since the test overwrites the golden values. This should never happen unless the regen flag is passed in.

The ideal solution is to not write anything to a file system however this is required because of the use of example tests for the general testing. I'm even more convinced now that that requirement is too hard.