Add do-block syntax for Semaphore acquire. Add Semaphore tests

[IanButterworth]

Add do-block syntax for Semaphore acquire

s = Base.Semaphore(2)
@sync for _ in 1:100
    Threads.@spawn begin
        Base.acquire(s) do
            foo()
        end
    end
end

Also, from what I can tell Semaphore wasn't tested in Base before this

[IanButterworth]

@tkf it just occurred to me that the do-block should probably return the return value of f(), in the same way that lock(f.. does..

I just pushed that and copied the code style from lock(f..