[issue] fixing the subsystem creation for unit test 

The modification of subsystem with init() makes it incompatible with the unit test. I have to make a temporary walk around to make the unit test work (https://gbmc-review.googlesource.com/c/dbus-sensors/+/12914/3/src/NVMeSubsys.hpp). 

I think the requirement of the init() function is debatable since it is similar to the start() function. 