Support for different SCID length

[obonaventure]

Since the initial packet can contain SCIDs of different lengths, it could be interesting to have tests with different values of the SCID length to check that remote servers can process connections from any SCID.

It could also be interesting to provide a plot of the ID length of the different servers

[mpiraux]

This use case raises two interesting points:

• A test can be constituted of several runs. In this case, each run tries a possible SCID length. For now, each scenario produce a single run. Maybe we can define several steps to complete a given scenario ? I have to be cautious to not clutter the interface with too many results and group them as part of a single scenario.
• There is no visualisation of the test results, but there is a high potential for using the test traces and present different graphs.

[mpiraux]

a44a129 adds a zero-length Source CID test, addressing a corner case of this issue.