Add test coverage to the stats shown by the message bot

[gkaracha]

This PR

• adds test coverage to our artifact (test-coverage.json), as reported by bisect_ppx
• extends the compare-stats subcommand to also print the test coverage difference between the two commits (usually base commit and HEAD).

I tried to reuse the infrastructure from #204 as much as possible, but there are a few differences between test coverage and the other two (gas costs and entrypoint sizes) which might be important enough to require us to treat test coverage differently:

1. it comes in percentages, not integer values (however we could consider them as integers, since they always have exactly two decimal points)
2. we have a separate entry for the total (which is meaningful), and it'd be nice for it to appear at the beginning or the end of the table.

I wonder whether it's best to store the percentages as strings in the artifact. Not sure.

[github-actions[bot]]

Gas costs: No change. Entrypoint sizes: No change.	Test coverage	4dc3d749f548eddf06f381bd87782956f9f376f6	8e242559bde831479b20323f9cc29c6f9c69def9	Diff
ctez.ml	None	100	100
parameters.ml	None	100	100
lqt.ml	None	100	100
cfmmTypes.ml	None	100	100
checkerEntrypoints.ml	None	100	100
ptr.ml	None	100	100
liquidationAuctionTypes.ml	None	100	100
fixedPoint.ml	None	100	100
common.ml	None	100	100
error.ml	None	100	100
mem.ml	None	100	100
checkerTypes.ml	None	100	100
fa2Interface.ml	None	100	100
kit.ml	None	100	100
constants.ml	None	100	100
cfmm.ml	None	100	100
burrow.ml	None	96.54	96.54
sliceList.ml	None	95.38	95.38
checkerMain.ml	None	92.86	92.86
avl.ml	None	91.05	91.05
checker.ml	None	87.1	87.1
liquidationAuction.ml	None	76.28	76.28
TOTAL	None	93.17	93.17

[github-actions[bot]]

Gas costs: No change. Entrypoint sizes: No change.	Test coverage	4dc3d749f548eddf06f381bd87782956f9f376f6	a901ad20bfdde6d021c76ac5d9c3d7b98d43a5ce	Diff
ctez.ml	None	100	100
parameters.ml	None	100	100
lqt.ml	None	100	100
cfmmTypes.ml	None	100	100
checkerEntrypoints.ml	None	100	100
ptr.ml	None	100	100
liquidationAuctionTypes.ml	None	100	100
fixedPoint.ml	None	100	100
common.ml	None	100	100
error.ml	None	100	100
mem.ml	None	100	100
checkerTypes.ml	None	100	100
fa2Interface.ml	None	100	100
kit.ml	None	100	100
constants.ml	None	100	100
cfmm.ml	None	100	100
burrow.ml	None	96.54	96.54
sliceList.ml	None	95.38	95.38
checkerMain.ml	None	92.86	92.86
avl.ml	None	91.05	91.05
checker.ml	None	87.1	87.1
liquidationAuction.ml	None	76.28	76.28
TOTAL	None	93.17	93.17