The sharding unit-tests' `kFutureTimeout` duration is too short for slow variants

The sharding unit-tests, which spawn asynchronous activities via launchAsync wait for up to kFutureTimeout = 5 seconds for these activities to complete. Some of these tasks however may perform database writes and other storage engine operations and on slower variants, such as DEBUG on arm64, this may take more than 5 seconds.

In order to make these tests more robust on slower variants we should increase this constant to 15 seconds.