Modified the tests to use mock functionality of get_child_port functi…

…on under portconfig utility (sonic-net#1464)

Modified the tests to use mock functionality of get_child_port function under portconfig utility. This will remove the dependency of port config utility in the sonic-buildimage and make the test case more robust specific to the DPB cli utility.

Signed-off-by: Sangita Maity <samaity@linkedin.com>