Closes #3087

For line-wrapping bytes were counted instead of codepoints. This
resulted in line-breaks being inserted at the wrong position and also
breaking UTF-8 characters.

Fixes #1022.

Ideally we could suppress the warning locally in the `UNIQUE_NAME` macro,
but that runs into at least 2 issues:

1) Clang actually does not consider the warning as coming from inside the
   `UNIQUE_NAME` macro, even though it correctly points to its expansion
   as the problem. This means that adding `_Pragma`s inside the macro
   around the __COUNTER__ usage does not actually silence the warning.

2) Adding the local suppressions anyway breaks the expansion of
   `MAKE_NAMESPACE` macro inside the templated test case macros. This can
   be fixed for the newest clang version by removing its use and using
   the uniqued `TestName` for the namespace name directly, but this breaks
   compilation on GCC, and older Clang versions.

Because of these issues, we introduce global warning suppression for
`-Wc2y-extensions` to be done with it. We should revisit this if Clang 23
fixes the local pragma based suppression, when it might be worth the effort
to rework the templated test case macros to support it.

Closes #3076

Not being able to filter generators to specific element has been regularly
causing problems. It was possible to use a dynamic section to run tests
for specific element in a generator, at least if the element had a nice
string representation, but the test case would still run once per element
in the generator.

With this change, it is possible to have the generator return only one
specific element, and do so based on the index, rather than the string
representation of the element. This enables simple debugging of tests
that fail for specific generator element.

* Removed `IGeneratorTracker::hasGenerator`, as it was never used.
* Removed `IGeneratorTracker::setGenerator`, as actually using it
  after the tracker was used would lead to inconsistent behaviour.
  Instead, the tracker is given the generator it guards during
  construction.