Add execution timing in code cell metadata for v4 spec

[davidbrochart]

No description provided.

[MSeal]

Probably a good idea to add a test that the timing values get assigned as expected in the resulting notebook document. And to check they have a format conforming (in case someone refactors timestamp).

[davidbrochart]

I added a test which checks for expected timing values and format. I think it's ready to be merged now.

[MSeal]

Thanks for adding tests