MAID-420 Crash in send tests

Adds mutex around std::future::set_value in two tests

This is another way of making sure promise won't get
destroyed by going out of scope before promise::set_value
finishes.

The tests are BEH_API_SimpleSend and FUNC_API_ParallelSend.
The proper way to do this is to pass ownership of the promise into the lambda. But seeing as this is only test code I'm willing to not care here.