Reorder imports in test_netsnmptestenv to be more logical
netsnmptestenv: Fix snmpcmd() error detection/exceptions

For one thing, we used re.match() to search for matches at the end of the
output, thus exceptions were never raid. Second, we only caught the case where
a MIB (and thus the OID) was basically known, but not available (ie. because
there was no subagent connected serving it) and reported it with the wrong
exception name. Third, we didn't catch the case when an unknown OID was
specified (eg. an unknown MIB or a known MIB but an invalid OID within that
MIB).
Add test cases for snmpcmd() exceptions to test_netsnmptestenv.py

These test cases cover the changes done in 706ecb06.
Add first test cases for netsnmpagent module itself

These test cases so far yet deal with rather low level issues of the module
only, such as the expected behavior after instance creation, whether a
connection to a master snmpd agent inside a net-snmp test environment set
up with the netsnmptestenv module could be successfully established etc.

More important tests such as the behavior of the different types of SNMP
objects are yet to follow. For this reason, the included new TEST-MIB.txt
so far gets used only indirectly (to test when it becomes available to the
test environment's snmpd).