selftests: xsk: Introduce test specifications

selftests: xsk: Introduce test specifications

Introduce a test specification to be able to concisely describe a
test. Currently, a test is implemented by sprinkling test specific if
statements here and there, which is not scalable or easy to
understand. The end goal with this patch set is to come to the point
in which a test is completely specified by a test specification that
can easily be constructed in a single function so that new tests can
be added without too much trouble. This test specification will be run
by a test runner that has no idea about tests. It just executes the
what test specification states.

This patch introduces the test specification and, as a start, puts the
two interface objects in there, one containing the packet stream to be
sent and the other one the packet stream that is supposed to be
received for a test to pass. The global variables containing these can
then be eliminated. The following patches will convert each existing
test into a test specification and add the needed fields into it and
the functionality in the test runner that act on the test
specification. At the end, the test runner should contain no test
specific code and each test should be described in a single simple
function.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210907071928.9750-4-magnus.karlsson@gmail.com