[tests] update reference solution due to changes in diffusion laws to

test/references/test_ff_rans2cni_komega-reference.vtu

test/references/test_ff_rans2cni_lowrekepsilon-reference.vtu

test/references/test_ff_rans2cni_oneeq-reference.vtu

test/references/test_ff_rans2cni_zeroeq-reference.vtu

test/references/test_ff_stokes2c_advection-reference.vtu

...ferences/test_ff_stokes2c_densitydrivenflow-reference.vtu

test/references/test_ff_stokes2c_diffusion-reference.vtu

test/references/test_ff_stokes2cni_diffusion-reference.vtu

...ndary_darcy1p2c_stokes1p2c_comparison_darcy-reference.vtu

...dary_darcy1p2c_stokes1p2c_comparison_stokes-reference.vtu

...ndary_darcy1p2c_stokes1p2c_horizontal_darcy-reference.vtu

...dary_darcy1p2c_stokes1p2c_horizontal_stokes-reference.vtu

...rcy1p2c_stokes1p2c_vertical_advection_darcy-reference.vtu

...cy1p2c_stokes1p2c_vertical_advection_stokes-reference.vtu

...rcy1p2c_stokes1p2c_vertical_diffusion_darcy-reference.vtu

...cy1p2c_stokes1p2c_vertical_diffusion_stokes-reference.vtu

...ndary_darcy2p2c_stokes1p2c_horizontal_darcy-reference.vtu

...dary_darcy2p2c_stokes1p2c_horizontal_stokes-reference.vtu

...y_darcy2p2cni_stokes1p2cni_horizontal_darcy-reference.vtu

..._darcy2p2cni_stokes1p2cni_horizontal_stokes-reference.vtu