Fix `-Wreturn-type` and `-Wunused-parameter` warnings in tests. NFC.

[Quuxplusone]

Note that or_else::[extensions.or_else] still has an unused variable:

auto failptr = [](eptr e) { return tl::expected<int,eptr>(tl::unexpect, std::move(e));};

but I didn't remove it because it looks like it probably indicates some missing test coverage.