Implement "Scrubber" mechanism for handling unstable output

[claremacrae]

Scrubbers are a way to write code that modifies the output being verified, before its contents is checked. It is typically used to remove unstable output, that might change between successive test runs.

This might be to:

• replace dates times values with some fixed text
• handle GUID values

[claremacrae]

As of 2020-04-26, there is an initial implementation of GUID-scrubbing on the master branch.

We are refining the interface on the Options branch, so the released API will differ somewhat from the initial implementation.