If a test case is not relevant to the testing environment (such as when
a NIC doesn't support a tested feature), the framework should skip it.
The mechanism is a skeleton without actual logic that would set a test
case or suite to be skipped.

The mechanism uses a protocol to extend test suites and test cases with
additional attributes that track whether the test case or suite should
be skipped the reason for skipping it.

Also update the results module with the new SKIP result.

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