Improve coverage of template functions

[GPMueller]

The baseline coverage is incomplete because unused template functions are not generated by the compiler. This can be fixed by instantiating templates in the unit tests. This should be all that is necessary:

template class FMath::detail::Field<FMath::Vector3, std::vector<FMath::Vector3>>;
template class FMath::detail::Field<FMath::scalar, std::vector<FMath::scalar>>;