add test case for #7976

[timotheecour]

• closes [typetraits] Tuple type name confusion #7976 by adding test case
• in reply to [typetraits] Tuple type name confusion #7976 (comment) from @@kaushalmodi , I verified that 0831292 is the commit that fixed [typetraits] Tuple type name confusion #7976 (see my corresponding PR revives: Move typetraits.$ to system. Fixes #5827 ; fixes #7976 #10071)

note1

I'm using the same technique here as I had introduced here #9927
it allows grouping tests in a single self contained file instead of splitting in multiple files (see also rationale here: see rationale here: #9927 (comment))

I've read this comment #9927 (comment) but testament doesn't support this, and the pattern is simple and gives fine grained control to test writer.

note2:

see tests/issues/readme.md for rationale why I'm adding this directory; my intent is to also use it for https://github.com/nim-lang/Nim/issues/10170

[Araq]

Issue numbers have no semantics, I don't like this at all. Extract the test file name from the issue title instead.

[krux02]

Things I don't like about this PR

• sneaking in a new file: tests/helper_testament.nim for no reason at all
• when defined(caseX) pattern
• calling the compiler to compile the test from the test
• No explanation in the test on what the test is actually testing

not joinable tests woul work equally well.
Then I am not sure if the behavior that is tested here should be engraved in stone.