selftests/xsk: move all tests to separate functions

selftests/xsk: move all tests to separate functions

Prepare for the capability to be able to run a single test by moving
all the tests to their own functions. This function can then be called
to execute that test in the next commit.

Also, the tests named RUN_TO_COMPLETION_* were not named well, so
change them to SEND_RECEIVE_* as it is just a basic send and receive
test of 4K packets.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230914084900.492-5-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>