[ransnc] Rename test problem, it is more a flat plate than a channel

test/freeflow/ransnc/CMakeLists.txt

test/freeflow/ransnc/flatplatetestproblem.hh

test/freeflow/ransnc/test_flatplate.cc

test/freeflow/ransnc/test_flatplate2c.input

test/freeflow/ransnc/test_flatplate2cni.input

test/references/test_flatplate_lowrekepsilon2cni.vtu

test/references/test_flatplate_zeroeq2c.vtu

test/references/test_flatplate_zeroeq2cni.vtu