Non-regression tests: skip tests instead of failing if link layer in PCAP is not supported.
Non-regression tests: skip tests instead of failing if link layer in PCAP is not supported.

This change is part of the work to add OpenBSD support to the library.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc