Add unit tests for pointwise tendencies

[nefrathenrici]

For the following tendencies, we will:

• replace cache variables with compute functions
• replace the state-altering tendencies with function(s) that return lazy broadcast expressions
• Unit test the tendency broadcast expressions
• Measure performance change on GPU

See the sponge tendencies for existing examples.

Tendencies

• horizontal_dynamics_tendency
• horizontal_tracer_advection_tendency