IPC::Fork::Simple Functional Tests

Currently IPC::Fork::Simple lacks unit tests but has a suite of functional
tests. These functional tests spawn (multiple) processes and have the
potential to hang (block forever on read) if there is a bug, so they are not
suitable to be run as part of "make test".

It is suggested the user run these tests by hand whenever they encounter a
potential bug in IPC::Fork::Simple or whenever working directly on the module.
Enhancements in the form of more functional or unit tests are always welcome.
