snapuserd_test: don't discard result of std::async

std::async returns a std::future whose destructor blocks until the
async function has completed, which defeats the purpose of using
std::async. The future needs to be kept alive to allow the function to
run concurrently.

Starting in C++20, std::async marked [[nodiscard]] to help catch this
misuse. Upgrading libc++ adds the [[nodiscard]] attribute to
std::async, so fixing this bug is necessary to keep the code compiling
after libc++ is updated.

Bug: 175635923
Test: treehugger
Test: m && m snapuserd_test
Change-Id: Id2e820248c2b6111aa843fb709e08a2c19677066