a few initial refactorings for Darcs.UI.Commands.Test

This is part of a larger series of changes I am working on
that will take longer to get ready to submit, but these
make sense on their own.

3 patches for repository darcs-unstable@darcs.net:screened:

patch 98a12cc5889764a0e9c813ddfdcecdc6c18fdd0f
Author: Ganesh Sittampalam <ganesh@earth.li>
Date:   Thu Jun  6 13:25:39 BST 2019
  * introduce explicit type for tracking test results
  This is in preparation for adding a new constructor to
  track "untestable" results.

patch 3f958feb66c0e6e1e1207b4d2faced9987772400
Author: Ganesh Sittampalam <ganesh@earth.li>
Date:   Thu Jun  6 22:53:00 BST 2019
  * introduce an explicit type to express the result of a strategy
  This is a bit of a brute-force/dumb refactoring to begin with.

patch 222a5f5fd12f84aa4e381f7c97c2447e6244c77c
Author: Ganesh Sittampalam <ganesh@earth.li>
Date:   Thu Jun  6 22:57:16 BST 2019
  * rename BisectState to BisectProgress

Attachments

• introduce-explicit-type-for-tracking-test-results.dpatch
• patch-preview.txt
• unnamed

* introduce explicit type for tracking test results
  This is in preparation for adding a new constructor to
  track "untestable" results.

Fine.

  * introduce an explicit type to express the result of a strategy
  This is a bit of a brute-force/dumb refactoring to begin with.

I like it. Factoring this out from the later changes eases review.

  * rename BisectState to BisectProgress

Fine, new name is more specific.