`TEMPLATE_PRODUCT_TEST_CASE` trips up `-Wsign-conversion`

**Describe the bug**
`TEMPLATE_PRODUCT_TEST_CASE`'s internals do something to trip up `-Wsign-conversion`, which causes problems for user code.

**Expected behavior**
No warning should be visible from user code.

**Reproduction steps**
```cpp
// g++-11 -Werror=sign-conversion
#include
#include

TEMPLATE_PRODUCT_TEST_CASE("hello", "", (std::vector), (int)) {}
```
[Compiler Explorer](https://godbolt.org/z/6ra1EoTen)

**Platform information:**

- OS: Ubuntu 20.04
- Compiler+version: GCC 11
- Catch version: v2.13.8 locally; trunk on Compiler Explorer