IS-04-01: test_19: Exclude MXL transports from interface bindings checks

[jonathan-r-thorpe]

Problem:

• Currently the IS-04 schema requires an interface_bindings array but does not enforce a minimum number of entries but does recommend at least one entry.
• However test_19 enforces at least one entry, making the test stronger than the specification.
• This means for MXL transport senders and receivers, this test fails, due to empty interface_bindings.

Solution:

• interface_bindings checks in test_19 are now ignored when the transport is MXL.
• This is in preference to weaking the tests for other transports, or restricting the check to only RTP transports.

[cristian-recoseanu]

This looks good.
Do we think other tests in IS-05 might also need looking at?

I'm thinking of maybe this one:

nmos-testing/nmostesting/suites/IS0502Test.py

Line 537 in a70a650

def test_12(self, test):