Add Variable bridges to use runtests without unbridged_variable

[blegat]

Some variable bridges don't support unbridged_variable and that makes runtests fail when getting ConstraintFunction or when getting ConstraintSet for scalar constraints. For example, ZerosBridge does not support it so it cannot use runtests.
Another example is the SOS cone: jump-dev/SumOfSquares.jl#353 I could support it in the nonweighted version but if it's weighted by arbitrary polynomials, it seems difficult to unbridge. It might be possible but it's not high priority. By using only vector cones in the tests, it turns out that only this small change is enough to pass the tests. This is still testing the inner model completely.

• Explain in docstring why this is used for
• Use it for ZerosBridge by adding a tests without scalar sets

[odow]

Did you try ZerosBridge? It still failed in a bunch of places

[blegat]

I did not try, I was just hoping it could serve as a test

[odow]

The issue is that a bunch of other places may call ConstraintFunction, so it is not so simple

[odow]

I don't think this is the right approach. Just write some other tests. You don't need general infrastructure.

[blegat]

What about now ?