A rudimentary framework for additional Python tests not run by default

* see the mail thread starting at
  <http://lists.freedesktop.org/archives/libreoffice/2014-February/059548.html>
  "Testing/Working on PyUNO?" for a rationale

* run the tests via top-level "make PythonTest_pytests" or "cd pyuno && make -rs
  PythonTest_pytests" or similar

* see the documentation in pyuno/PythonTest_pytests.mk for adding tests to the
  framework

Change-Id: I6a2a9e60b3294cd649f9cccbaffbd3f6bd79ecff
Test for fdo#74824.

The bug in question crashed LibO when inserting a group of cells.
This bug was quashed, per se, by commit
07e2c31831ad265b018e5fdf59bdde048fbb4d35, but it occurs to me that at
least the particular functionality of inserting a group of cells could
use more testing.

Change-Id: Icdbfff86fb0265eef325bcc94d9fc9f3e9e38413