Add a simple test file to test termination/non-termination

The purpose of these tests is to actually check with all kind of
mailboxes that the concurrent pipes terminate (or not terminate) as
expected when either the producer or the consumer stops.
This way we can detect these tricky regressions when the underlying
implementation changes.

Much more could be usefully tested in this library, but this behavior testing seems especially important.

You can try it like this:

```
cabal configure --enable-tests
cabal build
cabal test
```