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.
Merge pull request #44 from inetic/MAID-420-crash-in-send-tests

MAID-420 Crash in send tests