Add tests for delay lines

[simbilod]

Describe the bug
Some delay line components lay out a different length than the one passed as argument (see e.g. #909 for a quick fix to delay_snake3)

Suggested fix
We need tests specificlaly for delays that is independent of just tallying length, for instance compute the component area with c.area() and dividing by waveguide width.

More generally, we need more robust LVS, or some comparison of layout to design intent

[joamatab]

https://github.com/gdsfactory/gdsfactory/issues?q=is%3Aissue+is%3Aopen+lvs

#187

@damienbonneau
@tvt173
@sebastian-goeldi