Remove tests from .cabal file, adding a note that they are now in distributed-process-test.

Move out tests from this repository to a separate repository, called [distributed-process-tests](http://github.com/ps-labs/distributed-process-tests). This separate repository is proposed for inclusion in the `distributed-process` suite of repositories maintained by the haskell-distributed Github organization.

The rationale for this change is to make it possible to execute the various `distributed-process` test suites using any transport, not just TCP, in order to test that `distributed-process` does indeed work properly on those transports. Without this pull request, running the test suites over other transports than TCP requires duplicating the test suites, which is a hassle to maintain.

This split is similar to what is already done for `network-transport`, which sports a separate `network-transport-tests` package containing generic tests that can be reused by any instantiation of `network-transport`. The `distributed-process-tests` package contains a default instantiation of the test suites using TCP, so that `cabal test` in that repository works exactly the same as it does today in `distributed-process`.