Let net_dynamic_hb success with single node network.

[RicoGit]

Problem

net_dynamic_hb test fails every time one node left in the network. With this error:
thread 'drop_and_re_add' panicked at 'crank: network queue empty', src/libcore/option.rs:1008:5

That one may be related to the test (or the test net framework) and not the production code: Of course, there are no messages in the queue if there's only one node. A single node will always immediately deliver a batch whenever we provide input.

Originally posted by @afck in #385 (comment)

Proposition

The test or test framework should handle a situation when one node left in the network.