Use constexpr and hpx::wait_each in future_reduce test

[vaibhavj2006]

Fixes #

Proposed Changes

-Replaced #define NUM_THREADS with constexpr std::size_t NUM_THREADS for type safety and compile-time evaluation. -Updated test loops to use hpx::wait_each to handle futures efficiently as they become ready. -Improved readability and parallel execution behavior in the future_reduce test case.

[StellarBot]

Can one of the admins verify this patch?

[vaibhavj2006]

I think the CircleCI job ran out of allowed time