Extract boostraping code from convwatch.py to unotest.py. Use python builtin
unittest module as unit test framework. Specify the unit test modules in make
file. Another option would be to use discover mode of unittest module.

Add __pycache__ to global .gitignore to keep the source directory clean.
Another option would be to deliver the unit tests to workdir prior to test
execution.

Currently only system python3 is supported.

Change-Id: I2692817673f786e950e1176a17c7675f989755b6
Reviewed-on: https://gerrit.libreoffice.org/3214Reviewed-by: David Ostrovsky <David.Ostrovsky@gmx.de>
Tested-by: David Ostrovsky <David.Ostrovsky@gmx.de>