[test] Shorten multidomain tests, mainly by using very coarse discretization.

test/multidomain/2cnistokes2p2cni/CMakeLists.txt

...cnistokes2p2cni/test_2cnistokes2p2cni_boundarylayer.input

...in/2cnistokes2p2cni/test_2cnistokes2p2cni_reference.input

test/multidomain/2cnizeroeq2p2cni/CMakeLists.txt

...in/2cnizeroeq2p2cni/test_2cnizeroeq2p2cni_reference.input

...ultidomain/2cstokes2p2c/test_2cstokes2p2c_reference.input

test/references/2cnistokes2p2cni-ff-reference.vtu

test/references/2cnistokes2p2cni-pm-reference.vtu

...references/2cnistokes2p2cniboundarylayer-ff-reference.vtu

...references/2cnistokes2p2cniboundarylayer-pm-reference.vtu

test/references/2cnizeroeq2p2cni-ff-reference.vtu

test/references/2cnizeroeq2p2cni-pm-reference.vtu

test/references/2cstokes2p2c-ff-reference.vtu

test/references/2cstokes2p2c-pm-reference.vtu