rust-cpython: prepare for writing tests that require libpython
What I wanted is to disable the "cpython/extension-module<ver>" feature
while building tests executable, but that seems not doable. Instead,
this patch adds new features dedicated for tests.
The make rule is extracted so that we can easily run cargo tests.
Added a minimal test of ref-sharing as an example. More tests will follow.
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors