Some test cases or suites may be testing a NIC feature that is not
supported on all NICs, so add support for marking test cases or suites
as requiring NIC capabilities.

The marking is done with a decorator, which populates the internal
required_capabilities attribute of TestProtocol. The NIC capability
itself is a wrapper around the NicCapability defined in testpmd_shell.
The reason is twofold:
1. Enums cannot be extended and the class implements the methods of its
   abstract base superclass,
2. The class also stores an optional decorator function which is used
   before/after capability retrieval. This is needed because some
   capabilities may be advertised differently under different
   configuration.

The decorator API is designed to be simple to use. The arguments passed
to it are all from the testpmd shell. Everything else (even the actual
capability object creation) is done internally.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>