Rename test_netsnmpagent.py to test_netsnmpagent_init.py

We can only test the init behavior of the netsnmpagent module in this file as,
among other things, we explicitly test that calling agent.start() without
registering any SNMP objects does not make them available. So we can't at the
same time register and test SNMP objects -- calling shutdown() and
instantiating a new netsnmpAgent instance wouldn't work for the reasons laid
down in 9c6c5560.
Use __file__ instead of sys.argv[0] to find path to TEST-MIB

If run through the "nosetests" script, sys.argv will be /usr/bin/nosetests,
not the path of test_netsnmpagent_init.py. We have to use __file__ instead.
Re-import os and time once more in netsnmptestenv's shutdown() method

Especially "os" may have been __del__'d when shutdown() gets called a second
time by netsnmptestenv's own __del__() method.
Update TEST-MIB.txt to offer OIDs for more diverse testing
netsnmptestenv: Make snmpget() extract and return data and datatype

Calling functions will have no interest in extracting the data themselves.
netsnmptestenv: Add snmpset() method to set values