Ideas about splitting slow `l3build check/doc` into smaller even runs about l3build HOT 4 OPEN

To split l3build ctan, one possible way is to provide a pair of new options --no-check and --no-doc.

This part I think is a definite no: the entire reason that the ctan target is set up the way it is is that we want to ensure that a release build does 'everything' before getting to the .zip file creation.

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* Some new options like `--first-num <number>` and `--last-num <number>` which denotes the number of first and last tests to run seems a little better than `--first/--last`.

* Options that allow user to run the first third, second third, and last third (in general kth nth where 1 <= k <= n) is even better.

Sounds like --test-block=<m>:<n> or similar - 'test the <m>th block when dividing the tests into <n>' blocks?

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Could just be --block, so with a short version -b?

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To split l3build doc, assuming user knowns which names are specifically slow, for example the source3 in l3kernel bundle, latex3/latex3 repository,

* A new option `--skip <names>` would help a lot.
  It will opens the possibility of running `l3build doc source3` and `l3build doc --skip source3` in parallel jobs.

I'd imagine --exclude or --exclude-files, based on other tools: I guess one would likely go for just --exclude as we don't use file names, and this would need to apply to tests and to docs.

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