selftests/bpf: test_xsk: Don't exit immediately if validate_traffic fails

selftests/bpf: test_xsk: Don't exit immediately if validate_traffic fails

__testapp_validate_traffic() calls exit_with_error() on failures. This
exits the program immediately. It prevents the following tests from
running and isn't compliant with the CI.

Return TEST_FAILURE instead of calling exit_with_error().
Release the resource of the 1st thread if a failure happens between its
creation and the creation of the second thread.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20251031-xsk-v7-12-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>