Cleanup integration tests

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Followup to #946

Some integration tests are the leftover of early days when I'd test a half-dozen platforms in the same lab. Doing any-to-any interoperability testing like that shouldn't be netlab's task. Those tests should be removed, and if there's any value in them it should be transferred into the per-device feature integration tests.

We could however agree on a reference implementation -- an easy-to-get image that runs as many things as possible -- and add multi-vendor integration tests that would combine the reference implementation with the device under test.

Will be fixed in 1.7

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Making progress:

• VLAN: cdd9049
• VXLAN: 24a5d02
• VRF: 78d254b

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Cleaned up:

• OSPFv2: 0897de5
• OSPFv3: a3c46c6
• Core BGP: 8c2bde0

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Still missing:

• IS-IS
• MPLS
• BFD
• Gateway

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I'll eventually get them done ;)