Fix ProcessTestCasePOSIXPurePython to test the module from import when

_posixsubprocess doesn't exist rather than simply stubbing it out
after the fact.  This adds coverage for the RuntimeWarning as well as
using the pure python _create_pipe instead of using
_posixsubprocess.cloexec_pipe unintentionally with the pure python
code.

Ironically: I don't think any platform should ever actually _use_ the
pure Python subprocess code on POSIX platforms anymore.  This at least
tests it properly in this stable branch.  The pure python code for
this is likely to be removed in 3.3.