Improve error message when testing network factory

Currently, when network factory is under testing, but failed
without terminating the network factory. The mocked network
factory will stay registered and trigger another assertion
fail in teardown(). Thus, the test suite will only shows
the callstack that generated in teardown() instead of the
original fail. The error message is misleading and not useful
at all.

Thus, safely terminate and quit mocked network factory after
testing to prevent assertion fail in teardown().

Test: atest ConnectivityServiceTest#testMobileDataAlwaysOn
Bug: 175180558
Change-Id: I0f96332cc05221e576bd792c6cd26d9dccb4e228