fix Processes test failure for openjdk-src build with recent versions of OpenJDK 7

It looks like the UNIXProcess class has added posix_spawn support,
which is enabled by default unless overriden by the
jdk.lang.Process.launchMechanism system property.  For some reason I
haven't bothered to investigate, posix_spawn fails on MacOS, so this
patch sets the aforementioned property to "fork" to get the old,
working behavior.
Merge pull request #307 from dicej/posix-spawn

fix Processes test failure for openjdk-src build with recent versions of...