mpmc-queue: expand to 2R2W, 2R1W, 1R2W tests

I rewrote the test to allow a configurable number of readers and
writers, but then I figured out that our model-checker doesn't support
optarg() and its global optind properly, when using mmap()/mprotect()
snapshotting (it doesn't add the optarg library to be snapshotted...)

So, this test grows to 3 separate source files, at least for now. The
only change is the number of reader and writer threads.