Adding MultiProcessing() module to the runner remove call to prepareTestCase

[drake-mer]

Hello,

I have a a standard module for running my testSuite that do some initial configuration.

When I add the MultiProcessing() module to the list of modules to be used in a test session, the calls to prepareTestCase() that were made previously was not made at all.

I tried to track down the reason for this peculiar behaviour, but I failed to find a quick win solution by hacking some configuration flag.