Do not use initval in test model

PRs pymc-devs#7508 and pymc-devs#7492 introduced incompatible changes but were not tested simultaneously.

Deepcopying the steps in the tests leads to deepcopying the model which uses `clone_model`, which in turn does not support initvals.

tests/models.py

@@ -98,15 +98,14 @@ def mv_simple():
     p = np.array([[2.0, 0, 0], [0.05, 0.1, 0], [1.0, -0.05, 5.5]])
     tau = np.dot(p, p.T)
     with pm.Model() as model:
-        pm.MvNormal(
+        x = pm.MvNormal(
             "x",
             pt.constant(mu),
             tau=pt.constant(tau),
-            initval=np.array([0.1, 1.0, 0.8]),
         )
     H = tau
     C = np.linalg.inv(H)
-    return model.initial_point(), model, (mu, C)
+    return {"x": np.array([0.1, 1.0, 0.8]).astype(x.type.dtype)}, model, (mu, C)
 
 
 def mv_simple_coarse():