Reduce runtime of unit tests.

On, for example, this build, we have a failure due to time checks failing.

This MR speeds up slow unit tests which operate on data whose size can can easily be reduced. Some other unit tests, such as UnitTestMapperWireframer, UnitTestMultiMapper, UnitTestLagrangianFilter, LagrangianStructuresFilter, and UnitTestParticleAdvection are not amenable to this change. But this is a start.

As a nice side benefit, the unit tests took 320 seconds before this change, and now take 136.