Create unit tests for NETCONF connection status behavior change

Continue on https://git.opendaylight.org/gerrit/c/netconf/+/107036 and create unit tests that test the following scenarios:

1. when we are below max_attempts the device status is connecting

1. when we are exceeding max_attempts the device status is unable-to-connect

1. when device is turned off and on again we need to ensure transition from connecting to connected

1. when device is turned off and on again we need to ensure that device stays in unable-to-connect