[stokes][test] Prefix test with stokes

test/freeflow/staggered/CMakeLists.txt

test/freeflow/staggered/test_navierstokes_kovasznay.input

test/freeflow/staggered/test_stokes_donea.input

test/freeflow/staggerednc/CMakeLists.txt

test/freeflow/staggerednc/test_stokes2c_advection.input

...reeflow/staggerednc/test_stokes2c_densitydrivenflow.input

test/freeflow/staggerednc/test_stokes2c_purediffusion.input