Allow using only types in `TEMPLATE_TEST_CASE_SIG`. (#2995)

Right now `TEMPLATE_TEST_CASE_SIG` fails to compile when the signature contains only types:

```
TEMPLATE_TEST_CASE_SIG(
  "TemplateTestSig: compiles with two type parameters",
  "[template][onlytypes]",
  ((typename U, typename V), U, V), (int,int)) {}
```

The trick is to resolve the ambiguity between the two overloads of
`get_wrapper` (`TypeList` and `Nttp`) by making one match more strongly.
We also need to allow `reg_test` to register more than one type.

Add unit tests.

Fixes #2680

---------

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

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