Add support for TEST environments about l3build HOT 9 CLOSED

@benzea Does branch https://github.com/latex3/l3build/tree/testenv do what you want in terms of an environment form?

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\STARTTEST...\ENDTEST seems like an odd pairing: ConTeXt has start...stop and LaTeX has begin...end. I'd say pick one.

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@blefloch Was based on the fact that we already have \START ... \END.

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@josephwright Yes, I am happy with that. In particular if the documentation has a note on how to flush the page out (I am happy to propose changes to latex3/articles/lua-test-suite.tex).

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@josephwright Ah, I forgot about that. Then what you had is fine (what you have now too).

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I guess I'll see what @FrankMittelbach feels about the naming.

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OK, we'll stick with \BEGINTEST then: probably more natural. I'll look to do the documentation part too and merge into master.

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