[test][3p] Replace unnecessary dgf by input file

test/porousmediumflow/3p/implicit/grids/infiltration3p.dgf

...orousmediumflow/3p/implicit/grids/test_3pniconduction.dgf

...orousmediumflow/3p/implicit/grids/test_3pniconvection.dgf

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

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

...porousmediumflow/3p/implicit/test_box3pniconvection.input

test/porousmediumflow/3p/implicit/test_cc3p.input

.../porousmediumflow/3p/implicit/test_cc3pniconduction.input

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