selftests/xsk: declare test names in struct

Declare the test names statically in a struct so that we can refer to
them when adding the support to execute a single test in the next
commit. Before this patch, the names of them were not declared in a
single place which made it not possible to refer to them.

Signed-off-by: Magnus Karlsson <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>