plot tests: update tests with relative time stamps
plot tests: update tests with relative time stamps

commit 043fdefbc47567e7ad5d3fb9993d7c61d9ab369b
requires two changes to the tests:
- x values are floats instead of timedelta objects (line 1229)
- ignore the axes attribute of subplots which is just cosmetic and not about data (all the rest)

eerimoq/cantools,cantools/cantools