Improve test suite about gradualizer HOT 6 OPEN

I think it should be pretty strait-forward to come up with a test case for most of the uncovered lines. Most functions are helper functions for either the subtyping relation, for type_check_expr (type inference/propagation) or for type_check_expr_in (typechecking against a known type). Basically every erlang expression can exist in a context where we expect a certain type (e.g. a function with a spec) and in a context where we don't.

If some code is really hard to understand why it is there, perhaps it shouldn't be there or it needs some renaming or explanation in comments. Post a comment on any part of the code and I'll be happy to try to answer.

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SWEET! That's super useful @zuiderkwast!

For posterity I just want to say that the coverage report can be found in cover/index.html after running make tests COVER=1.

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It is possible to generet a code coverage report with make tests COVER=1 if I remember correctly. (I implemented that in erlang.mk some years ago.)

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How do I know what tests need to be written? It's tricky to look at the coverage report and figure out what will cause flow to reach each branch. Is it a better idea to pick a big corpus of code and type check that and then minimize each failing case?

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make tests COVER=1 doesn't do anything since a long time. For a while, rebar3 cover was making the pretty html files, but now after the strong cut away from rebar, make cover produces only the coverage.xml file, which is not very human readable.

rebar3 do eunit -c, cover works for now, and the files this generates are correctly gitignored.

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I think there are plain text cover files generated under cover/. If you need the html files, we can just add an option to generate them in the Makefile. Also, if you don't want to download the covertool to generate the xml file, we can make that a separate target, so that make cover only produces the html and/or text files.

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