Add a seed for random inputs in UpwindBiasedGradient test

[dennisYatunin]

This PR adds a random seed to the convergence test for UpwindBiasedGradient from #2345. There have been several random CI failures since I added that test, which can be avoided by using an appropriate seed for the randomized inputs. It is unfortunate that the convergence order error is not negligibly small for all possible inputs, but it makes sense that there will always be certain seeds that lead to arbitrarily high errors.

• Code follows the style guidelines OR N/A.
• Unit tests are included OR N/A.
• Code is exercised in an integration test OR N/A.
• Documentation has been added/updated OR N/A.