gh-107782: Use _testcapi to test non-representable signatures (GH-109325)

gh-107782: Use _testcapi to test non-representable signatures (GH-109325)

Builtin functions and methods that have non-representable signatures today
will have representable signatures yesterday, and they will become unusable
for testing this feature.

So we need to add special functions and methods to the _testcapi module
that always have non-representable signatures.