selftests/xsk: add option that lists all tests

selftests/xsk: add option that lists all tests

Add a command line option (-l) that lists all the tests. The number
before the test will be used in the next commit for specifying a
single test to run. Here is an example of the output:

Tests:
0: SEND_RECEIVE
1: SEND_RECEIVE_2K_FRAME
2: SEND_RECEIVE_SINGLE_PKT
3: POLL_RX
4: POLL_TX
5: POLL_RXQ_FULL
6: POLL_TXQ_FULL
7: SEND_RECEIVE_UNALIGNED
:
:

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230914084900.492-7-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>