test/tool-option-parsing: switch from pytest to unittest

test/tool-option-parsing: switch from pytest to unittest

For me, installing pytest for libxkbcommon is a bit problematic, so I
end up skipping it which is not great.

Switch to unittest which is built in to Python. It's not as nice as
pytest but good enough in this case.

Note: I was too lazy to switch the plain asserts to unittest
assertions...

Signed-off-by: Ran Benita <ran@unusedvar.com>