backport SNMP unit test infrastructure from master
NMS-6832 Cleaner Snmp code
- SnmpObjId could not correctly handle oids with large unsigned int values
- Fix MockSnmpValueFactory and ProperitesBackedManagedObject to handle snmpwalk output containing text like
  Wrong Type (should be XXXXX): INTEGER: 0
- Extended JUnitSnmpAgent and JUnitSnmpAgents to have a 'forceMockStrategy' flag
- Changed the linkd Nms10205aTest test to force the MockStrategy to avoid problems with agents
  so as to take the socket reponsiveness and such out of the equation

Conflicts:
	core/snmp/impl-mock/src/main/java/org/opennms/netmgt/snmp/mock/MockSnmpValueFactory.java
	core/test-api/snmp/src/main/java/org/opennms/core/test/snmp/JUnitSnmpAgentExecutionListener.java
	opennms-services/src/test/java/org/opennms/netmgt/linkd/Nms10205aTest.java
fix toString to handle unsigned ints > Integer.MAX_VALUE
added tests to toString, compare and decrement
fixed unimplemented hashcode
further fix for comparing oid with big subids and small subids