Bug: grader report may display negative numbers without mandatory parenthesis around them

[letouzey]

Bug description

Let's consider a exercise about a function foo with at least one int argument.
If the tests about this function (e.g. Test_lib.test_function_1) may involve negative integers,
then the grader report will display these negative numbers without parenthesis. This leads to
incorrect code fragment like foo -6 in the report :

Computing foo -6  : 1 pt
Correct value 6

This isn't just a cosmetic error, it actually prevent a student from copy-pasting these
fragments unedited in an OCaml toplevel for debugging purpose (the actual situation
was involving a function with many arguments).

To Reproduce

Create an exercise about a function foo : int -> int and test it for instance with Test_lib.test_function_1 with at least one negative test case.

Expected behavior

An output such as : Computing foo (-6) ... (with parenthesis around the negative numbers)

Additional context

I've looked a bit at learnocaml source code, it looks like this issue is due to Introspection.print_value being too optimistic in its need_parentheses sub-routine. I'd be tempted to add - to the characters triggering the case `Decided true (around line 186 in introspection.ml).

[erikmd]

Thanks a lot @letouzey for your very detailed bug report!

I can reproduce it; I've opened a PR adding a test + your suggested fix,
but the CI does not passes yet, so I'll have another look ASAP.