Make tests less specific to a particular model

[mhauru]

#1173 improves our test coverage by changing a lot of tests that used only the models that implement

    s ~ InverseGamma(2, 3)
    m ~ Normal(0, √s)
    1.5 ~ Normal(m, √s)
    2.0 ~ Normal(m, √s)

in various ways, to use a more general collection of models. However, many other tests remain that still have a hard-coded assumption that we are using that particular set of DEMO_MODELS. For instance, some tests assume that there is a variable called x and that it's an observation, or that there is at least one observe statement in a model. We should clean these up and make them all fully generic, making full use of the (excellent) setup in TestUtils.Models with logprior_true and varnames and other such functions used to implement the ground truth of what we expect from any given model.