selftests: xsk: Refactor teardown/bidi test cases and testapp_validate

Currently, there is a testapp_sockets() that acts like a wrapper around
testapp_validate() and it is called for bidi and teardown test types.
Other test types call testapp_validate() directly.

Split testapp_sockets() onto two separate functions so a bunch of bidi
specific logic can be moved there and out of testapp_validate() itself.

Introduce function pointer to ifobject struct which will be used for
assigning the Rx/Tx function that is assigned to worker thread. Let's
also have a global ifobject Rx/Tx pointers so it's easier to swap the
vectors on a second run of a bi-directional test. Thread creation now is
easey to follow.

switching_notify variable is useless, info about vector switch can be
printed based on bidi_pass state.

Last but not least, init/destroy synchronization variables only once,
not per each test.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329224316.17793-12-maciej.fijalkowski@intel.com