sdbusplus testing add new_mock method
sdbusplus testing add new_mock method

This method returns an sdbusplus::bus::bus using the mock
provided and adds the necessary EXPECT_CALL ahead saving
a line of code in all unit-tests using this.

Change-Id: Icc07405eb0bd4e4dca6c28fdb71e7ff2e5c06c3f
Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus