Tests for source-filter

[garethsb]

As per https://github.com/Streampunk/sdpoker/blob/master/checkST2110.js#L264, it would be good if test_10_83_2 and test_10_83_3 tested that source-filter was present and correct.

[garethsb]

@sparkpunkd @andrewbonney How many of the possible forms of a=source-filter: allowed by RFC 4570 Section 3 do you think we need to check for?

Do we need to allow <filter-spec> of excl as well as incl?
Most likely, <nettype> must be IN, but should the test allow <address-types> of IP6 or the wildcard * as well as IP4? Should FQDN addresses be allowed as well as numerical addresses?

Is this defined by ST 2110 somewhere?