Add some tests for Netflow decoding

The tests are a bit more expansive than the existing tests for sFlow
or NFv5 as we check the whole structure. I am also testing the
String() function as it is easier to read. It is a bit redundant, but
checking only for the wire format makes it difficult to compare with
Wireshark. Only testing for the textual representation is not totally
good as it is not what is used by users of the decode function.