selftests/xsk: add option to run single test

selftests/xsk: add option to run single test

Add a command line option to be able to run a single test. This option
(-t) takes a number from the list of tests available with the "-l"
option. Here are two examples:

Run test number 2, the "receive single packet" test in all available modes:

./test_xsk.sh -t 2

Run test number 21, the metadata copy test in skb mode only

./test_xsh.sh -t 21 -m skb

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230914084900.492-8-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>