[3p] Unify main files

test/porousmediumflow/3p/implicit/3pniconductionproblem.hh

test/porousmediumflow/3p/implicit/3pniconvectionproblem.hh

test/porousmediumflow/3p/implicit/CMakeLists.txt

test/porousmediumflow/3p/implicit/infiltration3pproblem.hh

test/porousmediumflow/3p/implicit/test_3p_fv.cc

test/porousmediumflow/3p/implicit/test_3p_fv.input

test/porousmediumflow/3p/implicit/test_3pni_fv_conduction.cc

...orousmediumflow/3p/implicit/test_3pni_fv_conduction.input

test/porousmediumflow/3p/implicit/test_3pni_fv_convection.cc

...orousmediumflow/3p/implicit/test_3pni_fv_convection.input

test/porousmediumflow/3p/implicit/test_box3p.cc

test/porousmediumflow/3p/implicit/test_box3p.input

test/porousmediumflow/3p/implicit/test_box3pniconduction.cc

...porousmediumflow/3p/implicit/test_box3pniconduction.input

test/porousmediumflow/3p/implicit/test_box3pniconvection.cc

.../porousmediumflow/3p/implicit/test_cc3pniconvection.input