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>