plot: added failing test for bug
plot: added failing test for bug

bug added in commit c7f5fa463b9c2f23d69c304c5fcd44c32d9b4ef3.
The tests passed because the changes worked as intended
but matplotlib (which was replaced by a mock) is not able to handle timedelta objects.

Therefore I have added a new test script which does not mock matplotlib.

cantools/cantools,eerimoq/cantools