Emulate connection to updates.jenkins.io to validate

[clarsonneur]

Sometimes, updates.jenkins.io has incoherence between plugins declaration and plugins packages downloadable.

This happens when a new version of a plugin appear. It gets declared but not found as package.
So, Jenkins updates reports an issue and suggest to retry later.
jplugins normally detect this and ignore that version until the package is downloadable.

This has been normally fixed by some PR. But it seems that the issue is already there.

So, if we could have a test case which help to identify that the issue is really fixed, could be better for regression or development validation.