Split out pre- and prove-commit tests to a new file

[anorth]

Splits the sector pre-commit and prove-commit tests out of miner_test.go to miner_commitments_test.go, to make them a little more navigable. They both used shared harness and helpers that are for now still defined in miner_test.go.

This is a pure re-organisation of top-level and some t.Run() methods.

• [x] Decide whether to land via squashing or merging, and be careful to do so correctly

[codecov-commenter]

Codecov Report

Merging #1404 (5774ba4) into master (9e636e6) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1404   +/-   ##
======================================
  Coverage    69.6%   69.6%           
======================================
  Files          72      72           
  Lines        7690    7690           
======================================
  Hits         5356    5356           
  Misses       1443    1443           
  Partials      891     891           

[anorth]

I did this with naive copy/paste, but with a bit of effort I can probably preserve most of the commit history on both files. I'll try that too.

[anorth]

Ok, this branch now preserves attribution on both test files, following a method documented here. You can't see it in the GitHub diff view (which squashes), but you can in the commit history.

It's a bit fragile though, relying on a merge commit in the history. I don't think it will survive squash-merging, or possibly even a rebase. I can recreate it if it's lost in a rebase against master prior to landing.

Since it won't survive a squash, we have to chose between:

• landing via merge: non-linear commit history, with some commits that don't build (due to duplication)
• landing via squash: lost attribution on one or the other of miner_test.go or miner_commitments_test.go

@ZenGround0 I'll take your preference here.