Run tests sequentially.

Tests are time sensitive. Running the tests concurrently can slow them
down enough that threads using threadDelay would wake up later than
expected, thus changing the order in which messages were expected.
Therefore we run the tests sequentially by passing "-j 1" to
test-framework. This does not solve the issue but makes it less likely.

The problem was first detected with
'Control.Distributed.Process.Tests.CH.testMergeChannels'
in particular.
Merge pull request #2 from tweag/run-sequentially

Run tests sequentially.